i have often wondered the same thing. the mounting bracket fits but i think it doesn't fit because of the way the ultimate seat is built up in the back. I have never tried it on the bike but i have put the bracket on the ultimate lowrider seat and just trying to get the passenger seat to fit on the bracket looked like it wouldnt. Again i have never really tried it on the cycle but i don't think it will work.
sorry. i am like you. rarely do i have a passenger and don't want to shell out the bones for a seat that i may never use.
if anyone knows if i am wrong please pipe up!
blade