Interesting concept, by the microwave magnetron has to be powered by an electrical source, which means a generator. I suppose that a fire could start the water boiling until it could turn a generator to power a the magnetron, but the laws of thermodynamics chimes in and kicks over that sand castle with resistive and thermal and mechanical losses.
I always thought that nuclear power might make steam work. I would think it could heat water the fastest. Collisions would be an issue.