Hi DanMan32,
I have heard that coolant draining is done in Siberia from coleagues at work who grew up there. The problem is not so much to do with engine cooling, The Siberians removed the coolant from the engine to avoid cracking the block when the coolant freezes at 50 below 0 F overnight.
If there was a plugin heater in the Prius coolant thermos, then the Siberians would not need to drain the coolant from the engine. The Prius would do that for them. And one would think it would be easy to keep the coolant warmer than 20 below 0, in the thermos.
As far as the need for coolant, my guess is its needed in the sub 20 below weather in rural areas. The roads might require slow driving, and sometimes allot of engine power to get through drifts and high snow on the road. Again, that is at slow vehicle speed. The iron engine block in these cars have a cavity where the coolant normally is. This cavity can act as insulation without coolant. The only way for heat to get out of a typical car engine if not by the coolant is by the thin web of metal at the top of the block and bottom of the cylinders, and the oil in the oil pan.
Cars also do not coast well in cold weather as the rubber tires are stiff. So, the engine is always pushing. I imagine after a couple hours at 20 below of slow driving on snowy roads, the engine is going to be close to overheating.
If the car is never going to be driven at a temperature above 20 below, I imagine one could make the engine without a cooling jacket and exposed to the outside. Then conduction cooling might work. Of course these cars would be useless for three quarters of the year in Siberia, and all of the year most other places.