I wonder if anyone has any good ideas for what you would do if the tf2 version of the source engine supported the Linked_portal_door entity.
It allows you to basically link any part of the map to another part without the player noticing the teleportation. Let's assume that you can shoot through it. Both projectiles, hitscan and area attacks.
Obviously this does not currently work in the game. I wouldn't expect it to get into the game unless Valve wants to change the system requirements.