Only thing I know of that may work for you is one of those heater body suits (i think that is what they are called). They are like a sleeping bag that you take into your stand and bundle up in. When the time comes, you unzip it and it slips off of your shoulders so you can get the shot off. I think they are pretty expensive and would be a pain in the butt to get into the stand, but they may be the answer you're looking for.
If that don't work...try layers.