Well, the cameras tilting mechanism is an important reason not to simply buy some other camera system (at least for me), so an extra digital system for the lenses seems much less desirable.
One thing I have failed to understand, maybe you could enlighten me: how can any digital back be synchronized with an unmodified SL66 at all? Is it enough to trigger the back some time before the shutter release, and does the back use a longer "internal exposure time" (for want of a better word)? That would mean any back could be adopted, given the mechanical problems (sensor thickness, width of the back - not all modern backs are that wide, are they?) could be solved and some cable release with included electric trigger was made. I think I am simply missing some basic understanding about how digital backs are operated