Thanks for the help! I still wonder if Mamiya ever made a grip for the RB though. I have checked several places and have not seen anything like it made by Mamiya. I am starting to think that the flash bracket might be the way to go. Problem I am anticipating is that it would have to be a tall flash bracket so it won't get in the way of the pop up viewfinder and wide so it won't interfere with the focus wheels. It will probably weigh as much as the RB...

Thanks for the link Pragmatist! That bracket seems like one I currently own for my old 645J. Not a bad bracket but it would have to be twice as big to fit the RB.