I just had a look at the hands again and it was one of those "what the heck was I thinking" moments. The joints are positioned very badly, so it's no surprise that the mesh looks weird when grasping. The extra geometry that's supposed to support the bend isn't were the bend actually takes place.

I'm also debating whether I should rewire the palm and try to establish an edge loop that follows the life line, so that there's a natural place for the mesh to fold when doing a thumb grasp. The other two main lines are pretty much there already.

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