Dunno. You might be able to do something with the eventhandler "hit". Even if it doesn't return the ammo, it does return the loon that fired the shot so you can then detect the weapons he has and then run a repair on the zombie if required. (setdammage 0.1 or whatever)
Alternatively, it might be easier to run eventhandler "fired" on the loons likely to shoot at zombies and do it that way. Or perhaps a combination of the two.
Or, if you're an addon maker, hardcode this stuff into the zombie himself, if that's possible.
Sorry, not very helpful.