Hmm, it's a bit out there, but totally possible...
I'd recommend disabling registrations on all forums (yes, all) then have a registration system in place that'd insert the new user data into all of your forums db's at the same time, so when a user registers at one site it spawns to the rest...
Now, as for updating such information, could get kind of messy...
You could also try editing the registration script on one forum so it'd spawn info to the others... again could get messy...
