I have the same problem with an "attic" room and here's how I plan to solve it
I will build a hole in the wall to put the air condx in so that the BACK of the unit, where the air comes out, will expell into the attic area.
I will build a plenum between the wall and the back of the air condx, so that the INTAKE air, which blows over the heat exchanger, can be taken from the garage below. In this way the cooler garage air is taken in, and the heated air is expelled out into the attic, where a little extra heat doesn't matter.
Of course, the FORWARD heat exchanger will take in room air and blow out cooler air into the room, as in normal operation..... In this manner, the air condx will work jusst as if it was mounted in the window--
--- with the REAR heat exchanger intake taking in cool air and blowing hot air out the back...
Sure, it is a bit of trouble, and requires a few sheets of sheet rock or plywood to build the plenum, and an opening in the garage (or porch) ceiling, but I can use a little $68 dollar 5200 BTU air condx from Home Depot to cool a 12 x 12 room, and have a 5 year warranty, and pay no penalty for efficiency, and not piss off my neighbors....
Besides, I've retired, and have nothing else to do but think up stuff like this...
Good luck,
Andy