I'm assuming that you're thinking of building a gravity flow system here. If not then you're still going to need power to run a circulator.
Assuming you're doing the gravity system: then get a standing pilot boiler and a power converter that'll handle the 120V needs of the boiler (very minimal without the circulator). On a good sized car battery you should be able to get a day or so of run time.... if not you can always charge the battery back up with the car.
Do lots of research as to how to plumb a correct gravity system.
If allowed by code, you might be able to use a standing pilot water heater as your heat source.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.