I have used bladder and hose in winter and if I keep it under shell clothes it does not freeze. Frequent drinking and to blow hose free from water after each zip also helps keeping it from freezing.

If I could trust a battery heating system like you suggested it might be practical in some situations. I could carry less insulation, but if the battery fails I would be in a worse situation than without it so I would not use it. I agree with others that hands and feet are more of a problem to keep warm than the torso.