I'm still planning to document the circuitry on my site, sorry it's been so long. I wanted to wait until I had my guns hooked up to my cab. Howard C has expressed interest in writing a wrapper for Mame that takes care of the software end thru DirectX, so anyone could hook up a hacked force feedback controller for rumble.
Dave Widel's keyboard LED driver on my site only supports 1 gun, but it supports rumble and both gun LEDs for that gun, which is really cool. Someone with a little C programming skills could hack it to support rumble on both guns and just skip the LEDs...