Hi Aris
I mean you shoot at the zombie in its body or limbs and it don't get any damage, then you shoot him in the head and bang! it died.
Half life (They Hunger) does use this system (Doom 3 ,much more advance game does not..you shoot a zombie in its leg and it died, really annoying).

But i am thinking..is it possible to attach another model to the first model , the one powered to the AI system? The second model is simple ..the head , that is vulnerable,while the first model, the zombie body ( the one who walks, fights ) is nearly invulnerable.
Player has to defend himself by the attack of the zombie body model and has to hit the zombie (head model) in order to kill the hideous creature.



In this way the illusion could be perfect without too much effort.
So, the body.mdl has the action with meele combat , the head has a simple "if is hit then health=health-500 and if health =0 then body.mld play death animation.
The problem is that I am a newbie. I got a lot of ideas but too much trouble to realize them.