That's an interesting idea, and I'm sure with a little more tweaking, it could become balanced.
I like the idea of being able to spawn anywhere on the map as long as humans can't see you, L4D style, but in the ZS gameplay, it's a bit too OP unless it had VERY LITTLE HP and did LESS damage than the headcrabs, in which case, it'd be easy kills towards humans, which also makes it OP.
In other words, I think this is a really cool idea, but I don't see how it can be tweaked to be completely fair.