Your best bet is having your member system read from the threads database. There is a membership integration hack at ubbdev if you want to take a look; it'll allow you to hijack the member routines from the ubb and use it elsewhere on your site.
I choose UBB over your custom solution because the UBB can and will change upgrade to upgrade, having to re-update your hack each time to do something with a new feature that isn't setup will have to require re-coding on your part.
