[thetwistedpanda]

I was going to write something up similar as well, but guess you beat me to it. Here's what I was going to do if you want to run with it - it should require less resources.

When defining a zone, create a trigger_multiple and hook StartTouch / EndTouch. An ADT array for each client, and when entering a zone push the index/entity/whatever and then remove when leaving (to account for multiple zones). Alternatively hardcode a limit of 31 zones and use bits but that's meh. It removes the requirement for position checks, which could be quite terribad if you're doing it OnGameFrame or every 0.1 seconds for all players on top of whatever the zone is deciding.