I suspect the devil is in the details, here. Doesn't the sensor have to fit right up against the film rails, in order to maintain the proper flange to image plane distance? I don't know how much room is needed around the edges of a sensor, for electrical connections or mechanical considerations. The only real "no-go" that I can imagine would be a lack of physical room. Apart from that, there are plenty of full-size sensors around; and surely there are people who can deal with the electronics. So I think it's mostly about getting the sensor to fit.
Me, I'm thinking of getting the Leica R to Nikon rear lens flanges, and just shooting my Leica R glass on my Nikons, both film and digital.