Hmm, I tried it, and it didn't seem to work. I might just save two versions of the dress, a constrained one for poses with the arms down, and an unconstrained one for poses with the arms up and for when one actually wants things to slip. That should cover the most common cases, so that folks will only have to fiddle with the group editor if they want special effects.

-- I'm not mad at you, just Westphalian.