"...My main problem is the tube freezing..."

I am far from a cold weather guy, but I do hang around hot weather from time to time. I always take a slurp, then blow the water in the tube back into the bladder, keeps it from getting too hot. I suspect that it might keep water in the tube from freezing too. No water, no freeze. Can't help you re the water in the bladder itself tho. Seems to me that carrying it under the jacket, and those little handwarmer gizmos, might do the trick, but that is just an uneducated guess...