I know this will probably be removed but I thought of an alternative type of gloryhole and one which could be done in a hotel room.
Hide under a sheet (like a ghost) but with the eyes and mouth cut out so you can see him and "recieve" him but he can't see what you really look like (which is the idea of a glory hole?)
You could open the door like this if you are discreet but would look silly in my view.
Or, just wear a full face mask. There is no way I can see anything working whereby you can't see who you are servicing, unless an accomplice is there to let them in as Kay says.
Even wearing a face mask is not foolproof as he could whip it off at any time.
All a bit daft in my view anyway.