The film-to-flange distance of 35mm rangefinders is in the range of 28mm, that of SLR's in the range of 40 to 46 mm.
So how would that work even with mirror-up? What did I get wrong?
(Edit: There are 35mm-rangefinder lenses designed for the use with an intermediate mirror-box. These should work with some adaper on a 35mm-SLR. But this is beyond the orginal question.)