I don't know Perl well, but as long as you are on NT, you can use ASP, which is a good idiot language for people like me

Heh! I've programmed in several languages over the years, but I can never seem to learn Perl (could be that I've known the others since I was a kind, now at 30 I'm way too old to be learning new crap

).
I use a product called ASPUser (which ironically enough, I believe can be gotten at aspuser.com). I use this for quite a bit of NT validation work and did something very similar to this. I also do the reverse and use the UBB members list to password protect databases and otherwise in ASP.
You can use ASP with this software to hand-build the member files with ease (hmmm...I've got a library somewhere...though not sure if its going to work with UBB6...I only verify against my 6 board). If it were me, I'd remove all the registration / login features from the UBB...which ya can do from the control panel. Set up the ASP to build these files with some large hashed password (you don't want anyone learning the passes - though they can probably do that through the cookie file as its in fricken plain text...what the hell were they thinking).
From here, it would be VERY simple to do. Practically no hacking to the UBB itself (maybe can do it with none at all)...just have ASP file build out cookies for the login and forward ya on to the board before signing in.
I'm probably not any more help on this subject other than a few hints, but I hope this helps
