There's all kinds of strange adapters floating around Ebay. I believe the last one I saw of interest was an adapter to mount a Hassy on either my Nikon or Canon. I'm going to have to look for that one again.

It would take some way to stop the lens down in order for a lens to work, or in the case of an RB, you would have top manually hold the lens closed. Any machinist worth their salt could fabricate a board with a holding mount out of aluminum. It probably would cost you more then you'd want to pay tho to get an image that wouldn't cover anything better then 6x7. Any excess coverage by the lens would be to insure peak 6x7 frame performance.