Wow this sounds like a great mision idea , I love playing around with the zombies on the mission editor. I do like the idea of finding certain things/activating radios around the island to aid in an escape/call for help.
A few ideas/suggestions for you =
Make every round count (Perhaps it could be made that weapons come with limited ammo and perhaps at one place there will not be weapons for all, so survival would depend onworking together).
Perhaps a "gunstore" or a few around the map (Buildings with weapon/ammo crates inside), cars having random amounts of fuel (nothing like runing out of petrol in the middle of nowhere surrounded by zombies ).
Perhaps a few slots for police (which start armed at a police station) a few army (National guard or something with humvees and trucks) and the majority being unarmed civilians (at first anyway) .
Oh and will you place the zombie cures around the map or will you do it so it is upto the player to decide if he wants his friends to kill him before its too late... or savour his time left with no care for his friends safety! Muahahaha !
Cheers and I look forward to playing the map, Col.