Illusion of space

snip

Don’t think so. I guess the best you can do is make a teleporting hallway or something to create the illusion.

Camera screen with teleporter?

Easy. Right when you reach the second floor, you teleport to an exact copy of the room that has the hole in the middle. Then, when you go back down the stairs, you teleport to the stairs of the original building.

That’s exactly what I would have done in Marathon. I had no idea you could do that in hammer.

You can. It would be difficult to make seamless.

In marathon I would have settled for the teleport in and out graphics and sound, it’s a lot less noticeable in source.

Though no matter where you’re stood on the staircase when you’re teleported, your destination will be the same. You could minimise it by making the staircase so thin that there’s no space for you to move side to side.

Setting up a landmark for the teleport correctly would eliminate this problem.