Frank, is that a devere 5x7 back? I've heard many tales about 5x7 OEM toyo but have never seen one (even in pics). Very cool whatever that is. Always thought it would be nice to upsize, although the investment in new lenses and enlarger kept me from looking too hard.
I really like the camera. I've had mine (4x5) since the early 90s. It needed a new bellows immediately. The replacement one still has that new car smell.
For a roll film holder, if try it, I may just get the standard kind everyone's mentioning. But I might make a Frankenback like what you're describing to mount a dslr for some harmless fun.