Thanks for you comments. I am however not sure if it could work that way, because a prop wouldnt morph with the figure carrying the cloth.

I have thought, maybe I can compute two or three vertex positions (one projection on left leg, one projection on right leg, one projection on hip) and make a weighted sum of these depending on the distance.  Also I have some idea for the calculation itself that I want to test.

Will keep you informed, but it will take a while. This week and the next will be busy.