to be honest this seems sort of easy to me... I would setup spawn points where the zombies would be allowed to spawn through out the level. Then I'd have the player emit a c_scan. All the spawn points would be some sort of invisible block or something and when scanned it would check if:

A) It can see the player
B) If it's to far away (B could be eliminated by adjusting the c_scan.

I'm going to be using something like this for my RPG. I will have a bunch of spawn spots through out my level, or just 1 spawn spot per room.