If you (I) have a propane furnace and let the OWB go out and just heat the house with the propane furnace, wouldn't the pipes to and from the heat exchanger pick up enough latent heat to keep them from freezing when circulating?
Otherwise, this is coming from an outsider point of view and I may be all wet--Couldn't one wrap the lines in the house with an electric heat tape (or two or three) to put at least enough heat back into the circulating water to keep it from freezing?
Finally, (this is from the farmer in me) what about a farm stock tank heater? They are 1500 watts. They make styles that float in the tank or sit under a raised tank but the non-floaters aren't as efficient. Mine is a floater in a 120 gallon water tank for our horses. It's an open tank with no insulation. Last night it was -8*F and absolutely no sign of freezing. They are only $20 and they are set to maintain water at 40 or 45*