Sailor, I have often dreamed of putting a very similar concept in place. Great idea. Amazing that it is not more prevalent considering the cost efficiency.
I also want to make a point about the water in the heating tank or water jacket of the double broiler setup. If you are stirring the milk in the main pot, there is very little reason to circulate the water. For two reasons:
1. If the water is being heated from the bottom, it will rise (because it becomes less dense) and create convection currents that will circulate as effectively as any other method. Just drop a little food color in and watch. If you were heating from the top it might be a problem, but definitely not from the bottom.
2. The milk is also absorbing the heat as well and then distributing it throughout itself as you stir so there is a constant flow of heat by conduction from the water underneath, through the pot, to the milk, where it is distributed and then it moves back to the water if there is colder water somewhere in the outside container. By this stirring of the milk there is a kind of equalization of the temp overall. My water tank stays at the same temp throughout with no circulation and a 2500w element on the bottom.
Just a personal observation. Next to newton's laws, my favorite subject was thermodynamics.