I've been playing around with the trigger_camera entity (see a couple of maps in 'Unfinished Stuff').
On the entity page for trigger_camera, it states that the player needs to be in the area of the camera or it doesn't draw the area. That, I believe, is an error. trigger_cameras appear to work just fine with the player at one corner of the max size map Hammer allows, and the camera in the other corner in a completely separate room (no func_walls between).
The camera just needs a target. If it's not the player, just stick an info_target in the room with the camera, name the info_target and set that name as the target for the camera.
The tutorial on cameras also makes a similar statement about having to teleport the player into the camera area. I don't believe that's at all necessary.