Interesting thread idea.
From my running experience, I know (read several times) that the lungs can cope with temperatures as low as -5Â° Celsius. Below that, it gets tricky. For a lifter it is even more difficult, as we make lots of small breaks during training whereas the runner's cardiovascular system is working nonstop. Lots of supersets are the key here, I suppose.
You could also sip occasionally some hot beverage, which you brought in a thermos flask outside.
Some more thoughts:
A free-mounted radiator would be nice in a garage gym.
Sweating in the cold can lead to a chill pretty quick for some guys. A towel is probably a must for quick-sweaters, as is a cap.
If working out with a training buddy, one should pay attention to have both guys work out at the same time, to avoid the aforementioned cooling.