You can try to look in this thread:
http://www.jboss.org/modules/bb/index.html?module=bb&op=viewtopic&t=forums/ haven't tried it yet, but I'm hoping this will help, because I want to do a similar thing.
Been there, done that. Still no luck.
The file is no longer auth.conf, but login-conf.xml. It appears that he changed the order of the attributes from one to his last post, perhaps that is a clue. But I doubt it.
Good luck to you. Please reply to this thread with your solution if you get it working.