That's not a bad idea. I'm trying to get something working like portal so a hole in the wall linked to one in the floor might be kinda weird.
solution to recognizing if it's in the hole is to have an invisible entity attached to the hole that uses collision detection only with the player to know if it's within the right area to run the code. as for making the wall and floor work properly, I could make it so that only the one wall I am currently in will be passable until I go 50%+ through the hole and then the other one will be passable.as for the visuals I was going to add a 3d view entity on the wall and hope that would work for both sides.
If they have the same texture though, I don't know how to do that, since I was thinking of using a trace funtion to see if there was still enough space around the hole based on the texture and which ones would be shootable.