Hah! I had a quick 10 minute stab at the neck hydraulics and getting an effective mouth rig. Oh dear! They both suck! Rather than open in a scream, the jaw geom looks more like it`s un-attaching from the rest of the head! Rubbish!
I`ve started blocking in the tongue, and got some of the springs to work. The idea with the springs is that they look more like tendons when they strech and add a more complex motion to a simple one.