Screw the alien. It's all about being in the jungle, creeping along slowly.... Oh, a bunch of marines! Leap a couple storeys into the tree, creep along, jump down inside their defensive perimeter.... *kill**kill**jump* Nope, they didn't see me...
If you model characters as points, you could probably use BSP trees for the scene geometry. You might get weird things with characters appearing entirely or not at all when they're peeking around corners, but it would be efficient.
This is all if you're crazy enough to want to do this.
Sort of... except that in your world, a counterstrike server, in addition to managing the state of the game and handling communication with all 32 clients, now has to do a lot of the occlusion processing for each client.