That bridge is horrible, if you're looking for an honest opinion.
Not that I want to turn someone off of Sphinx, I want the world to use it. But, right now it can't be everything to everybody, unfortunately.
It sounds like you need a forum for an existing WP site. Have you considered Vanilla or bbPress? They both integrate just fine with WP 2.1, heck... I've even integrated them both with MU.