steebu, good explanation. But I believe that they should be able to assign target points or procedural changes to the animations so that (in your example) the hand would always touch the head.
Adding procedural animations to canned animations could lead to things like a successful steal/block/rebound attempt always having contact between the hand and the ball, players adjusting their arm/hand position based on their relative proximity and angle to the ball/defender/backboard/rim on both offense and defense. It could also lead to an incredible variety of animation possibilities.
They already have what looks like some of it in the game, I think. The other day I had two players going for a defensive rebound with a player on offense behind them both. The offensive player grabbed the rebound sideways over the two defenders and the arm farthest from the ball was clipping through his own head as he reached for the ball.