I've got to agree there. I've used a lot of different forum softwares over the years, and the only one i can suggest that would even remotely help would be YaBB
The ONLY reason i suggest YaBB is beacuse all of it's data is kept in flat files and not in a database, you can login to the system using calls to the YaBB main script from your PHP scripts.
EG:
YaBB.pl?action=login&user=username&password=ABC
This would mean that you'd be in control of the database in it's entirety, and as long as the user names an passwords where registered in YaBB correctly (EG: YaBB.pl?action=register&username=user) connecting it is not too difficult.
The bad news is that the software is written in Perl not PHP and it does require some savvy to install it.
Cheers
Shawty