I'm thinking the Heatercraft would be tapped into the pressure side of the engine coolant, so it would be in series with the hot water heater (but you'd probably want to have the Heatercraft first in-line). I'm thinking two barbed couplings and a 4 hose clamps would make your connection.

The first time you run it, the engine would fill the hoses and heater element, and after running it you'd have to top off your engine coolant again.

Am I missing something and being naive about air locks?