Jeremy,
What I had done (on a temporary basis on the back of the house) was slide the "stationary" slider glass inward to leave an opening. It's not desigend to be opened from that side but after removing a few screws I was ables to do so.
I then cut a piece of melamine the height of the doorway and the width of the AC unit +3" on each side. Then I closed the "stationary" door back to meet with the melamine so it was held in by the frame channel and the door. I added some 2x4 bracing to act as a shelf for the unit. Worked great.
Perhaps you could slide one side of a window open and utilize a similar approach. Thet even make skinny AC units called Casement Slider AC units starting at 14" wide
http://www.discountmaven.com/phpdocs/ajtest/item_list.php?dept_id=00033 . Sears, Wally World and most other electronics outlets carry them.