Could that just be handled with flavour text? The zombie takes an injury and the description is that their head is gone?
Maybe they could have the flaw:
Fragile Form
Upon taking a Injury condition, the Zombie player rolls a die.
On a result of 1 - 5, the Zombie has lost its head (it is treated as if blind and cannot turn people into zombies).
On a result of 6 - 11 the zombie loses a leg (it collapses and is relegated to crawling speed).
On a result of 12 - 17 it loses an arm (loses half strength modifier?)
On a result of 18 - 20 the zombie is actually cut in half. The bottom half (the legs) die, but the torso and head remain a danger (albeit a static one).
It could be cumbersome, but it might just give the right feel. It would be better, too, if the PC attacking the zombie rolled the dice (thus letting them use HP's to affect it, and making it a little more interactive as well)