So the upshot is: place a passive heater at a low point with a straight shot to the inlet of the engine block. The outlet has the tstat. Right?
In this case, there is a coolant pump for the heater core that pumps coolant through the block when the engine is off, which happens a lot in the Prius. This gives me the option of running that pump and a passive cooler to force-circulate the warm coolant prior to engine start. The best place for the passive heater would be the return hose from the heater core as close to the engine as practical. Insulating the hoses would help too I suppose.
The last question I have to answer: Is there a coolant flow valve for the heater core? If so I will have to hack that too.
Luckily, I have one hint as to the direction of flow...
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