There's a hole in the wall. Drop your teleporter pyramid on the ground, then toss it through the hole. Teleport to it with the other one, gg. Stairs go to a trap door behind the bartender.
- through trial of telekinesis stat and gear I can conclude BS on that idea, although a conceptually good one - not true
Solved -
When you press the button, a small trap door is revealed in the north eastern corner of the inn, which I believe can be set to hidden again by pressing the button a second time making it deceivingly hard to find as people press the button trying to discern what it is doing without being able to see upstairs
