I still have a Mamiya 645 and a set of lenses. I’ve used them on an EOS 5D Mark III with a Fotodiox Adapter with focus confirmation chip, which came off, luckily outside of the camera.
I reverse mounted the 80mm lens, also with extension rings and it worked fine, albeit a bit clunky, also because aperture is fully manual which adds a few steps for each shot. Well, I also have a Canon EF100mm Macro lens which is much easier to use. Maybe it’s an idea worth considering to buy an original macro lens or a set of extension rings instead of spending some effort for a kludge which could make you cave in and buy the macro lens anyway…
Find my current setup here.