It's been done more or less, and it just wasn't fun. I've attached my admittedly very crummy map, Zombie Dome, which was my attempt at such an idea. Granted, I didn't run this in INF with a timer, so it did get even more boring than it otherwise probably would, but the problem with this kind of map is that there is very little action on the part of the player. You end up just sitting and spraying the mob, and that's just not fun.
