I am in the same boat as John McCaskill. Northwest Houston with a 15,000 btu window AC unit I rigged up into the overhead of my garage. I have ported ducts through the attic the the awning outside the house (one each for supply and discharge air). It works great, not visible from outside the garage, and in about 30 minutes it makes the garage much more tolerable to work in on a hot day.
I have however insulated the ceiling in the garage, and I have a radiant barrier and foam board insulation on the garage door (which faces west and catches a good amount of sun)
Grady - "Thelma, we found Dean's finger"
Thelma - "Where is the rest of him?!"