Source -- though I'll grant that the temperature is much lower than at the exhaust. But the cooling system offers a more efficient gatherer of heat from the engine, than just gathering heat from the exhaust.
Depends on the goal, obviously. If you're just trying to use the combustion heat more efficiently, that's a different story.
I think use of heat pipes works well in that environment, though.
Then we can mount a cooling tower in the back, make chilled water and hot water for an air handler and control everything with pneumatic controls.
This makes me wonder if developements in this type of technology may modify/improve the energy contributions of something as simple as wood stoves. Jotul ought to be at least examining this.