getting parameters from j_security_check form

I'm working on an app that uses JNDI Realm authentication, and I'd like 
the user to be able to pass additional parameters through the login page 
(say, a third field besides username and password).

I think it's not possible to do this through regular container-based 
authentication.  Is it possible to access an extra parameter in the 
j_security_check form using SecurityFilter?  If so, how?  Any help is 
appreciated...

Thanks very much,
Chris



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