Actually, there is a Japanese guy who tried to convert a Polaroid SX-70 to accept roll film, but his results aren't good enough yet. See his pictures here It's a very interesting experiment, though, and we're in the process of rethinking the construction options to see if there is another way around it. It may or may not be feasable, we're not sure yet. All depends on whether we can tame the electric circuitry into behaving as we want it to. It would be nice to end up with a working foldable fully automatic 6x9 SLR!
Problems of Shinsaku are the position of the roll film holder - it needs to be built in deeper inside the body. Film transport is far from ideal as well. We're thinking about using a Rada roll film holder (like the one you used), which would improve film transport. Electric circuitry needs to be rewired as well around the camera body and not across the holder for ease of use. But this is a far more complicated modification than yours, which, I must say, looks really elegant. Touché!