The problem with this method is that if you pin your IKHandle to your Head for example, rotate your head, then blend the Pin back to the Body, your Hand will actually "move". Instead of simply giving making the Head no longer control the Hand, it actually moves the Hand into the default position where it was constrained to the BodyRoot (if that makes sense).
I had this same issue with head locks too, and eventually gave up. Can anyone help me with this? Really what seems needed is that when you switch from Head Pin to Body Pin, the offset is adjusted for the body pin constraint. THAT i believe is the problem. The fact that the Hand is moving from the position of one constraint to another, instead of actually just letting the constraints tell it where to go, the hand is letting the constraints tell it where to start as well..
Hard stuff to explain for me. Hopefully you can help. Thanks!