[stoatwblr]

Looking at that image, are you sure there's a OEM valve in the heater line? I haven't seen one in use for decades because of their tendency to fail (usually in the "closed" position) or leak coolant into the cabin. I've had various older cars which have done either or both (and no heat when there are snowdrifts around is "no fun")

Modern cars tend to use flaps to control airflow through/around the heater matrix. This means your "heater blocked" bypass setup is unlikely to be needed.

Thanks for posting the ideas though. I've been toying with something similar to achieve fast warmup of both the engine and its WVO feed.

Does the Prius suck water from the block to the thermos or does it simply use a larger water capacity?

(FWIW I've noticed that manufacturers have tended to try and reduce water capacity over the years and I always assumed it was to achieve faster warmup - less water = less specific heat and modern designs recover vented coolant rather than dumping it.