I got the impression ZBrush wasn't an option here. If it is an option I think using the Morph Brush might be more reliable than projecting one mesh onto the other.
I'm not a C4D user so could be completely off the mark, but this workflow on cgsociety seems to describe something similar to Veneri's suggestion. Looks like they use Pose Morph Tag, Morph Deformer and a weight map?