Just a thought - put it in a separate tank that is insulated and keep that hot and then let the water pump circulate it with the heater hose to the block when the engine starts - less energy loss this way and you should be able to really heat up the water and dump it to the engine when it starts as well as the heater if you want some cabin heat plus you preheat the water with the engine for start and stop driving during the day in case you can't plug in. A few pound of extra water for much better warmup. I think I need to do a test with a bucket of hot water in the car to see how it cools off.
The Prius has the same contraption. After 8 hours, when i start up the next morning, the cooland temp after pumping back into the engine is 55C, 120F. Wish I could find one in scrap yard.
Just put a valve on it so that you can close it if it is colder than the engine until the engine has extra heat to heat it up with. The other thing is you can heat it up with less power and loose less heat if heated with a block heater and use less power to keep it warm. Then you dump it all into the engine just when you need - better than heating the engine with a block heater because the engine is not as well insulated.