how about creating a control file dynamically?
You can use any method you want like
shell script to create a controlfile 
or
sqlplus to create a control file.
I have used the second method.
i am just passing a value to the control file..if you are looking for a sequence number from database...we'll have to think about it.

----------------------------------------------------------------------

first lets truncate the table (since i am inserting data)

----------------------------------------------------------------------

mag@mutation_mutation > truncate table member;

Table truncated.

mag@mutation_mutation > exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

----------------------------------------------------------------------

lets run the scriopt
it reads the value entered and generates the control file..and invokes sqlldr session to load the data

----------------------------------------------------------------------

bash-2.03$ loadmember
Enter the constant to be loaded
1847

Generating the controlfile member.ctl as

load data
infile 'member.dat'
insert into table member
fields terminated by ','
(
control_id constant '1847',
name,
address
)

SQL*Loader: Release 9.2.0.1.0 - Production on Tue Dec 23 10:07:48 2003

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Commit point reached - logical record count 4

----------------------------------------------------------------------

lets check wehter it has done the job

----------------------------------------------------------------------

bash-2.03$ sqlplus mag/mag

SQL*Plus: Release 9.2.0.1.0 - Production on Tue Dec 23 10:07:52 2003

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

mag@mutation_mutation > select * from member;

CONTROL_ID NAME         ADDRESS
---------- ------------ ------------------------------
      1847 Charlie       250 yonge st
      1847 Peter         5 Brookbanks dr
      1847 Ben           123 King st

mag@mutation_mutation > exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

----------------------------------------------------------------------

this is script 

----------------------------------------------------------------------

bash-2.03$ cat loadmember
#!/usr/bin/bash
echo Enter the constant to be loaded
read const
sqlplus -s mag/mag <<EOF
set feed off
set head off
set termout off
prompt
prompt Generating the controlfile member.ctl as
prompt
spool member.ctl
select  'load data'||chr(10)||
        'infile ''member.dat'''||chr(10)||
        'insert into table member'||chr(10)||
        'fields terminated by '','''||chr(10)||
        '('||chr(10)||
        'control_id constant ''$const'','||chr(10)||
        'name,'||chr(10)||
        'address'||chr(10)||
        ')'
from dual;
spool off;
exit;
EOF
sqlldr userid=mag/mag control=member.ctl
[b]
and data file i used
[/b]

bash-2.03$ cat member.dat
Charlie, 250 yonge st
Peter, 5 Brookbanks dr
Ben, 123 King st

#!/usr/bin/bash
echo Enter the constant to be loaded
read const
[b]sqlplus -s mag/mag << EOF[/b]
set feed off
set head off
set termout off
prompt
prompt Generating the controlfile member.ctl as
prompt
spool member.ctl
select  'load data'||chr(10)||
        'infile ''member.dat'''||chr(10)||
        'insert into table member'||chr(10)||
        'fields terminated by '','''||chr(10)||
        '('||chr(10)||
        'control_id constant ''$const'','||chr(10)||
        'name,'||chr(10)||
        'address'||chr(10)||
        ')'
from dual;
spool off;
exit;
EOF
sqlldr userid=mag/mag control=member.ctl

lets consider the same script with line numbers.

     1  #!/usr/bin/bash
     2  echo Enter the constant to be loaded
     3  read const
     4  sqlplus -s mag/mag << EOF
     5  set feed off
     6  set head off
     7  set termout off
     8  prompt
     9  prompt Generating the controlfile member.ctl as
    10  prompt
    11  spool member.ctl
    12  select  'load data'||chr(10)||
    13          'infile ''member.dat'''||chr(10)||
    14          'insert into table member'||chr(10)||
    15          'fields terminated by '','''||chr(10)||
    16          '('||chr(10)||
    17          'control_id constant ''$const'','||chr(10)||
    18          'name,'||chr(10)||
    19          'address'||chr(10)||
    20          ')'
    21  from dual;
    22  spool off;
    23  exit;
    24  EOF
    25  sqlldr userid=mag/mag control=member.ctl

----------------------------------------------------------------------

idea is to generate the contents of ctl file ( using Unix utilties like sed is more simpler, whole script will be 2-3 lines only.....but needs more understanding of unix tools and regular expressions).
i used sqlplus..to do the same..
i am  just spooling the "typical" contents of the control file
like 
	load data
	infile filename
	insert into table_xx
	...
	...
	..
meanwhile, i embed the value of the paramters ( whcih the user supplies)
and use this script to be used in sqlldr.

lines 5,6,7   -> deals with formatting so that no heading and feedback are returned from sqlplus
lines 8,9,10  -> just displaying some user friendly messages. equivalent to echo in os scripting
line  11      -> initiating the spool process. anything results after this line is writte to the file
lines 12 to 21-> just a simple query that displays things from dual. I am addding use supplied value in line 17
                 chr(10) will make the result to be displayed in the next new line.
line  22      -> terminating the spool process
rest  lines   -> exit and invoke sqlldr.

here is a much simpler version using sed

----------------------------------------------------------------------

demo

----------------------------------------------------------------------

bash-2.03$ lmem

Enter the constant to to loaded
7876

The control file to be used to load is

load data
infile 'member.dat'
insert into table member
fields terminated by ','
(
control_id constant '7876',
name,
address
)

invoking the sql*loader

SQL*Loader: Release 9.2.0.1.0 - Production on Tue Dec 23 13:12:04 2003

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Commit point reached - logical record count 4
bash-2.03$ sqlplus mag/mag

SQL*Plus: Release 9.2.0.1.0 - Production on Tue Dec 23 13:12:08 2003

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

mag@mutation_mutation > select * from member;

CONTROL_ID NAME         ADDRESS
---------- ------------ ------------------------------
      7876 Charlie       250 yonge st
      7876 Peter         5 Brookbanks dr
      7876 Ben           123 King st

mag@mutation_mutation > exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.1.0 - 64bit Production
With the Partitioning and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

----------------------------------------------------------------------

the script , control file and dat files used

----------------------------------------------------------------------

bash-2.03$ cat lmem
#!/usr/bin/bash
echo
echo Enter the constant to to loaded
read const
cp member.ctl temp.ctl
sed -e "s/$const/$const/g" temp.ctl  > temp1.ctl
echo
echo The control file to be used to load is
cat temp1.ctl
echo
echo invoking the sql*loader
sqlldr userid=mag/mag control=temp1.ctl
rm temp.ctl
rm temp1.ctl
bash-2.03$ cat member.ctl

load data
infile 'member.dat'
insert into table member
fields terminated by ','
(
control_id constant '$const',
name,
address
)

bash-2.03$ cat member.dat
Charlie, 250 yonge st
Peter, 5 Brookbanks dr
Ben, 123 King st

what do you mean by [i]some other stuff in UNIX?[/i] ??
i am using the sequence inside the oracle db.
for every sqlloader session or every time u execute this script, the sequence value is returned as CONTROL_ID

----------------------------------------------------------------------

demo
First I display the current records in table
and truncate them
then i run the script.
then i check the records again.

----------------------------------------------------------------------

bash-2.03$ sqlplus -s mag/mag @za

CONTROL_ID NAME         ADDRESS
---------- ------------ ------------------------------
        79 Charlie       250 yonge st
        79 Peter         5 Brookbanks dr
        79 Ben           123 King st

Table truncated.

bash-2.03$ loadm
the generated sequential value is 80

The control file to be used to load is

load data
infile 'member.dat'
insert into table member
fields terminated by ','
(
control_id constant '80',
name,
address
)

invoking the sql*loader

SQL*Loader: Release 9.2.0.1.0 - Production on Wed Dec 24 10:21:38 2003

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Commit point reached - logical record count 4
bash-2.03$ sqlplus -s mag/mag @za

CONTROL_ID NAME         ADDRESS
---------- ------------ ------------------------------
        80 Charlie       250 yonge st
        80 Peter         5 Brookbanks dr
        80 Ben           123 King st

Table truncated.

----------------------------------------------------------------------

script
im using a sequence from the db.

----------------------------------------------------------------------

bash-2.03$ cat loadm
#!/usr/bin/bash
const=$(sqlplus -s "mag/mag"<< EOF | perl -ne 'chomp;s/s//g;print;'
set feed off
set head off
select memberseq.nextval from dual;
exit;
EOF)
echo the generated sequential value is $const
cp member.ctl temp.ctl
sed -e "s/$const/$const/g" temp.ctl  > temp1.ctl
echo
echo The control file to be used to load is
cat temp1.ctl
echo
echo invoking the sql*loader
sqlldr userid=mag/mag control=temp1.ctl
bash-2.03$ cat member.ctl

load data
infile 'member.dat'
insert into table member
fields terminated by ','
(
control_id constant '$const',
name,
address
)

--
-- sample load. first parameter $1 is directoryName and second parameter $2 is filename.
-- during input, you need to escape the slashes (/)
oracle@kaapi#./somescript "\/temp" somedat.dat
Control used to load is 
load data
truncate into table t2
fields terminated by ',' 
(
c1,
c2  constant '/temp'
)

SQL*Loader: Release 10.2.0.1.0 - Production on Tue Jun 29 19:53:28 2010

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Commit point reached - logical record count 2

C1         C2
---------- ------------------------------
va1        /temp
va2        /temp

--
-- Second run. Just changing the name for the directory. Anyvalue can go in

oracle@kaapi#./somescript "\/u01\/mydir\/otherDir" somedat.dat
Control used to load is 
load data
truncate into table t2
fields terminated by ',' 
(
c1,
c2  constant '/u01/mydir/otherDir'
)


SQL*Loader: Release 10.2.0.1.0 - Production on Tue Jun 29 19:53:36 2010

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Commit point reached - logical record count 2

C1         C2
---------- ------------------------------
va1        /u01/mydir/otherDir
va2        /u01/mydir/otherDir


--
-- This is the script
--
-- somectl.ctl is the template control file.
-- For every load, a new temp.ctl is created and used to load
-- 
oracle@kaapi#cat somescript
cat somectl.ctl | sed -e "s/dirName/$1/g" > temp.ctl
echo "Control used to load is "
cat temp.ctl
sqlldr dbadmin/xxx control=temp.ctl data=$2
sqlplus -s dbadmin/xxx <<EOF
select * from t2;
Exit;
EOF


oracle@kaapi#cat somectl.ctl
load data
truncate into table t2
fields terminated by ',' 
(
c1,
c2  constant 'dirName'
)

oracle@kaapi#cat somedat.dat
va1
va2