I guess the block heater maybe the best option since you have played with lifepo4 batteries, maybe you have a few to make a pack to run an inverter to power the block heater?
Not sure how much the heater benefits since its my understanding or misunderstanding the prius pumps its coolant into a giant thermos under the fender at shut down to repump to aid in warm up at next use.
Think Ive seen online where some guys installed a heater in the thermos?
If you plug in kit has the wiring according to Enginer I know it wont work til the Ready light appears and anything spent to ready the system faster should pay you back quickly allowing use of the phev kit too on such short trips.