Mamiya make a 2 handed grip for the RZ intended for use from an aeroplane. I don't know if it would attach to an RB, but I am not aware of any reason why it wouldn't. The Mamiya web site also says you can attach stabilizers to it....

I can't comment on the relative ease of loading other MF cameras, but I agree that the removeable insert does make loading easier sitting down (I have both RB and RZ cameras). Have you considered extra backs or inserts?