As far as I know, there is no MF rangefinder which takes a digital back. As all the MF digital backs I have seen are rather bulky and protruding, it would be difficult to look through the viewfinder , as Ben Rubinstein said.
There were at least some MF rangefinders for which polaroid backs were built, like the Fuji 690 series, so you could possibly build or adapt a digital back for them.
I am no expert in this, but it seems to me that the problem photosensors have with light hitting at an angle might be related to their size, and as 35mm or APS sensors have smaller photosensors than MF backs or LF scanning backs, you can work with a smaller distance between optics and sensor in larger formats. But then, close optics - sensor/film plane distance in MF is larger than the complete thickness of a point-and-shoot anyway.
There are some specialist non-reflex MF cameras taking digital backs, from Seitz, Alpa, Gilde and possibly others, but they are not rangefinders.
Christoph