Norm, Rollex 6x9 roll film holders were made to fit the early 9x12 Linhof's, I have one, they don't fit any of my 9x12 cameras.
Rather than adapt a roll film holder for 6x12 it would be a lot easier to make a mask and shoot 6x12 format images on 9x12 cut film, using film holders, after all 9x12 film is still available. A 6x12 roll film back would be bulky and make the camera less flexible. One major advantage of the 9x12 cameras is their relatively small compact size compared to their 5x4 cousins.
Ian