Is the room completely open or is there a 'doorway' that they need to pass through?
You could put an invisible npc @ the doorway and set it's proximity to be a line / ellipse rather than a circle.
Code:
quest::set_proximity($x, $x, $y - 100, $y + 100);
If the room is an open-air room, you could bound the area by 4 invisible npcs, setting their proximities to create a square/rectangle around the room.
What about a hailable npc to get in, flagging the hailer, and using the boundaries set by invisible npcs?