Nope, I have never heard of one, you should be able to cover 6x7 and 6x9 with and RB lens, but I think you would not be able to cover 4x5 with an RB lens...I imagine if you got a hold of a junk RB you could remove the mount and make your own board with it.
Yes, the mamiya RB67 back fits on the graflex 2-1/4 X 3-1/4 graflock back. I asked the question on this board a few months ago and was told it fits, so I bought an RB67 back for my Century Graphic. It fits perfectly and works great. It is much better built than the graflex lever-wind back.
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.