I think I have isolated the problem, it seems to be the smoothScale channels in the hip2. Removing these channels cures the asymmetry. It is a mystery to me as to why the smoothScale channels cause the asymmetry. The lThigh_smooY and rThigh_smooY channels are identical except for the 'otherActor' line, and yet it looks like the channels are operating in the opposite sense to each other. The smoothScale channels were actually the first thing I suspected, but not finding any asymmetry in the channels themselves, I assumed the problem must be elsewhere.

As the purpose of the smoothScale channels is to improve the appearance of the actor when its neighbour is scaled, and as they seem to be having the opposite effect in this case, perhaps the smoothScale channels relating to the thighs should be deleted from the hip2.