Is your wood/coal hand fed boiler a pressurized hydronic system? I ask this because most codes require outdoor wood fired boilers to be unpressurized, i.e. open to the atmosphere.
My solution would be to pressurize the system AND use a very, very large expansion tank. Here's the concept. Heated water expands in volume. There needs to be a place for that water to go. That's the expansion tank. Properly sized you don't need to worry about a power failure. Here's how to size the tank. (1) Assume the boiler is cold, (2) Assume the boiler water is hot, really really hot the hottest it can possibly get with a full fuel load and lots of draft, (3) Assume all that hot water stays in the boiler, i.e. no working circulator. Now determine the increase in boiler water volume between (1) & (2). Calculate the expansion tank needed such that the pressure does not rise greater than 5 psi below the pop off pressure on the safety relief valve. The benefit of such a system is there is no water loss. When it cools or power returns it recovers automatically. You must use metal piping suitable for the high temperatures. On my coal boiler I frequently have 240 deg boiler water, after a full burn and the circulator pump stops.