If I was hooking one up for normal use when the engine isn't going, I'd put tees in the heater hoses right where the return goes into the water pump by the alternator, and where the heater line comes out of the head by the oil filter housing. Thing is by putting the coolant heater output into the head (where it should be, as the head should be warmed first) is the wrong place for when the engine is running, because as you say the flow is reversed once the engine water pump is spinning.
Not sure what the best option is, sounds like you're rather pressed for spare space as it is
