I believe it was Bicycling magazine that recently ran a story on this. As I recall, and I could be incorrect, they concluded that the 5 or so degrees that one oz of alcohol lowers the freezing point may not be worth the tradeoff of its effects on the rider. They suggested starting out with warmer liquid (as suggested in this thread already). From my military days, if it were me, and I had a jersey on under my jacket, I would consider keeping one bottle in a rear jersey pocket under the jacket, and swap periodically with the one in the bottle cage.