Mechanim and repositioning body parts help

I did a google search on how to lock off the position and rotation of limbs in mechanim and found that by deselecting the body parts in its body mask during the import session does exactly that, the problem i'm having is that the limb stays in the position of the muscle definition and no matter how I try to change the position of the characters arm in the muscle definition, it always goes back to the way it was and changing it in scene doesn't help either as the position of each transform in the arm section all move to the hand and the positions always revert back to their original positions upon pressing play.

What i'm trying to do is to keep a walking animation but to keep one arm in a rigid position when a torch is equipped so that the light doesn't go all over the place.

I managed to do this in legacy but i'm now making the transistion into mechanim as it is proving easier to link together animations.