So I found this during my last run

Doesn't look useful since i'm pretty sure it keeps you in the area of the staircase but that sure didn't stop me from getting excited about it.

Odd glitch. It's also possible to clip through the doorways of the Grand Staircase if you constantly spam the "call partner" button while swapping between characters to slowly push a third character through a door, but it's incredibly unreliable and doesn't seem viable to skip anything.