I probably haven't thought this through enough but...
How about copper tubing, a pump - like
http://www.amazon.com/gp/aw/d/B007XH...XN542E2NG370JE but not pressurized, just filled with water. If you turn off the pump on a longer drive, let it boil, then you just fill it up before the next time you expect it to work. Maybe there would be a more elaborate way of having a reservoir which would refill the system with the circulation pump the next time it's turned on, without having the reservior be part of the circulation circuit.
Edit: Maybe position the pump and reservoir as low as possible, so that with the pump off, water would drain back to the reservior, away from the heat exchangers. Sizing the reservior to be running near empty while the pump is on / system circulating