I have a freind who can in an emergency , heat his house using his hot
water heater by gravity. It is tied into the same piping as his boiler
is. If he looses power to his house...and thus the boiler, he turns
some valves to switch over the system to the hot water heater. It then
heats the water in his baseboards by gravity.
Would it work in your scenario ? Yes, but it would have to be properly