For some time now, I've been trying to search to find out if there is some way to have something that is approximately like an Alpa TC, but with one or maximum 2 lenses and a COUPLED rangefinder. Of course, the idea is that it should be easy enough to rig it together, even if one has to pull together parts from different systems. Originally, my idea came from seeing those press cameras like the Mamiya Super 23, or the Baby Speed Graphics that had coupled rangefinder on the side. However, then I realised that I might not be able to have the focal lengths that I would like to have (about 35mm - 47mm) or the focus accuracy. It is now starting to look like so much bother that perhaps I should just forget it and get a Mamiya 645 AFD instead.
One "problem" I might foresee with modifying a Mamiya 6 to be able to put a digital back on it is that putting your eye to the viewfinder may be quite uncomfortable afterwards - the digital back will be right in the way.