Actually, the MAILFORM itself can be considered as one big "user exit".
One of our list owners does what you want to do with the following
ADDREQ1 section in his   listname MAILFORM file.  This "calls" a simple
exec called "FUNIMAIL" (included at the end) and sends the :ADDADD section
to the potential subscriber.
 
   I make absolutely no claim about accuracy, usefulness, desirability,
compatibility,  nor whether this will work in 1.8a.  I know L-Soft has
announced some major changes in the way you can configure messages in
1.8a (for example you can turn off the cc: to the potential subscriber
of that message to the owner in ADDREQ1 and there are new ways for
list OWNERS (as opposed to LISTSERV postmasters) to control messages
without letting them get at MAILFORM files which are a real potential
security risk.
 
   Anyway, this has been working for us.     marty
 
 
Part of LNAME MAILFORM (LNAME and [log in to unmask] would be list name
and owner e-mail address of course).
 
REMEMBER:  Anything in single quotes is "interpreted" (executed or
substituted) by LISTSERV whent the mailform section is invoked.  Ie.
   'listname'  becomes the name of the list, etc.
 
*----------------------------------------------------------------------*
:ADDREQ1 Request for subscription to list 'listname'
Dear list owner,
 
  A request for subscription to your distribution list 'listname'
('title') has been received on 'LSVFDATE()' from 'username' <'whom'>.
 
  You can, at your discretion, issue the following command to 'myself' to
add this person to the list: ADD 'listname' 'whom' 'username'
.bb remark == 1
(do not forget the "PW=list_password" keyword)
.eb
 
   The potential subscriber will also receive a request for some
information to return to you before adding him or her to the list.
This request will be sent in a separate mail item.
 
'FUNIMAIL("ADDADD","LNAME",whom,username,"*FROM* [log in to unmask]")'
Virtually,
 
   The LISTSERV management
:END
*-----------------------------------------------------------------------*
*-----------------------------------------------------------------------*
:ADDADD  Request additional information...
.fo off
Dear networker:
 
   Thank you for your interest in the LNAME list, an electronic
forum for individuals employed in ... etc...
 
   Here you can ask your questions or whatever...
 
Please mail your response directly to me at any of these electronic addresses:
 
                        [log in to unmask]
   ... etc. ...
 
:END
*-----------------------------------------------------------------------*
 
 
 
FUNIMAIL EXEC on LISTSERV 191/A disk:
 
/* Dummy function call to LSVIMAIL as a function - ie. in MAILFORM */
/* NOTE: Specifically for 7 or less arguments... */
 
   parse arg a1,a2,a3,a4,a5,a6,a7
   Call LSVIMAIL a1,a2,a3,a4,a5,a6,a7
   return ''