I'm not too sure this would be worth it. If you drive on a cold day, the window glass and the skin of the car don't get warmed very much by the heater. Basically the heater just heats the air, which represents a very small amount of heat. So I think that keeping the engine warm is the sensible goal.
I wonder if you could do this with the engine oil? If the car had a dry sump lubricating system then it would be easy to replace the oil reservoir with a thermos. There are racing car suppliers (e.g. Pegasus) that sell the stuff needed to convert a car to a dry sump system. http://www.pegasusautoracing.com/advcat ... yID=OILING
Look at the Accusump stuff and the oil tank heater...
I wonder if you could do this with the engine oil? If the car had a dry sump lubricating system then it would be easy to replace the oil reservoir with a thermos. There are racing car suppliers (e.g. Pegasus) that sell the stuff needed to convert a car to a dry sump system. http://www.pegasusautoracing.com/advcat ... yID=OILING
Look at the Accusump stuff and the oil tank heater...