I think there is an option for a 6x6 back on an RB. I've been thinking about the same idea and I believe that with the P-adapter coupled with an M-adapter (vertical or horizontal) you could then attach one of those S-shaped multi-format film backs (6x4.5, 6x6 & 6x7) (6x4.5, 6x6 & 6x9) originally intended for the Mamiya Press cameras. With these backs, make sure they have the thin metal masks for 6x4.5 and 6x6. I'm at the point where I have everything but one of these multi-format backs. I even have the 6x4.5 & 6x6 masks, although, I doubt I'll use the 6x4.5 format. I'll keep you updated with my findings.

Marc