ah, thats too bad. i was hoping to not write my own system.
have you ever considered making a forum support light version that made it easier to support different forum software?
it would not have any login functionality or user management. it would instead only verify authentication via the particular forum's auth method.
is user authenticated via forum?
send to forum login form
if you moved most of the features back to the forum to handle, you could probably support much more software.