Ah right, I suppose liquid and gaseous water indeed ought to react the same way to microwave radiation.
Damage to the magnetron? I hadn't even considered that. I was just thinking "something catches fire".
The "mason jar full of grass clippings" is a personal anecdote from when I was a kid, an event which thankfully didn't cause a house fire, but sure made a stink. I'm pretty sure what happened is the metal rim of the mason jar created sparks which ignited the dry grass. Of course, a mason jar is probably actually safer than if the grass were loose in the microwave , since (unless it shatters) it's going to choke the fire with its own CO2. I wonder, how airtight is a microwave? They usually have fans in them, so presumably the answer is "not airtight at all".
What about my "recycling the microwaves" idea? Using photoelectrics (does it count as photoelectric if the photons are microwaves?), either to actually recover some of the wasted energy, or just to detect how much is going to waste, perhaps using that information to shut off the magnetron, or to somehow intelligently change how the microwaves it emits are distributed?