Former antenna engineer here. I had a passing interest in knowing how safe I was in my work. Unfortunately, research in the field resulted in very much the kind of answers that are presented here. One researcher would present a paper saying, as long as you're not getting warm, your fine. Another would say were already all dead. Microwaves ARE harmful. It's a matter of degree. Tissue heating and damage to DNA were two modes of harm. There was a third I can't remember at the moment. I've never really understood why tissue heating was considered harmful if it wasn't, you know, burning you, but scientists seem to think so.

Bottom line is, I think more research is needed on the subject. Until then pretty much every country has rules about the power level that is safe for humans. I had to routinely calculate the minimum safe distance from a radiator for testing and installation purposes. They also have rules about the amount of power allowed to be radiated into ones head. This is called SAR (specific absorption rate). In the US it is the FCC that regulates these values. You can look them up there.

My advice, until someone can clearly provide a super accurate answer, I would err on the side of caution. Whenever I can, I use the speaker phone on my cell phone to keep it away from my head and when I get to where I'm going, I take it out of my pocket and keep it nearby, but not directly against my skin (power level drops significantly with distance). And I would NEVER voluntarily get warmed by a microwave device. But, if you have a liberal attitude toward your children being born with flippers, you do you.

Yes, I always warm up my hands on cold mornings in the microwave.

One word. Cateracts.

Look up neutron bombs.

Microwave ovens work on resonance, so I'd be worried about heating up parts of the body more than others. Without resonance you wouldn't have effective heating so it would be a bit complex to warm all parts of the body equally.

Raytheon, Epirus and other manufacturers already have "human microwave heaters" and they're mobile, except that they're called "Active Denial System".

Short answer: yes.

The Human Heater from Silicon Valley.

One of my grandfathers used to work for Nortel. One of the projects he worked on was the Trans Canada Microwave, which was a microwave relay system built in the '50s to carry television and telephone signals across Canada. The towers were installed all over Canada in remote locations and high elevations. Maintenance on the system could be required even when the weather was bad. My grandfather told me that the engineers who worked on the towers would sometimes stick their hands in front of the microwave emitters to warm them up. It's anecdotal, but I'm relatively confident that it's theoretically possible to warm people with microwaves.

Big caveat, though. Those engineers knew how powerful the emitters were, they knew that microwaves are not ionizing radiation and thus posed no cancer risk, they knew roughly what percentage of their hands was composed of water, and thus how much heat energy their hands would absorb from the emitters at a given power level. That's the only reason they were willing to do it, well that and they were probably the kind of people who got a kick out of doing something that would appear insane to most of the populace.

It seems very unlikely to me that a microwave system could be turned into a safe people-heating system for at least the following reasons:

• Feedback loops. All modern HVAC works on feedback loops. Your thermostat detects that the temperate is cold, it fires up your furnace / heat pump / electric baseboard / whatever and produces more heat. When the thermostat detects that the temperature has reached the set-point, it shuts off the heat. Current thermostats would not be able to detect the effect of microwave heating, which prevent the establishment of a feedback control loop.
• Uneven heating. Things with more water will heat up a lot faster than things with little water. This is usually fine when microwaving food since most of our food is water, in varying concentrations. If you're heating up a burger in the microwave, you can put the patty in by itself for a minute, then put the bun in for 15 seconds, then reassemble a burger that doesn't have a cold patty or a stiff overcooked bun. If you're heating up a person, you can't ask them to take out their almost-entirely-water eyeballs to ensure they don't overheat.
• Failure conditions. If your heat gets stuck in the on setting, the maximum result is probably that your house will get sweltering hot but not hot enough to kill you in a moderate timeframe. Depending on power levels, a microwave heating system could internally cook people in their sleep if it entered a failure mode where the heating got stuck in the on setting.
• Efficiency. It takes a considerable amount of power to run even a small microwave, and that's blasting microwaves into a relatively tiny cubic area. Trying to heat people would require microwaving a much larger volume, and said volume would also be moving around. Trying to emit microwaves in even a house-sized volume would probably be prohibitively costly.
• Interactions with metal and other objects. Microwaves can create intense electrical fields around metal objects, and those can become intense enough to create plasma and electrical arcs. Hell, you can create plasma in your microwave with two grapes. Blasting microwaves into a large volume with unknown contents would be a great way to create an unexpected fire.

TL;DR: Yes, but the ways in which it could go wrong if done incorrectly mean it is essentially guaranteed to be deemed never worth the risk.

Microwaves don't heat food "from the inside" necessarily, but from a depth that the microwaves manage to penetrate into it. In the cases of the microwave frequencies used in domestic appliances, that translates to around 4cm, give or take a small amount. So you would theoretically be able to heat your subject subcutaneously down to that depth. Note that the surface would also get heated in the process. There are fancy formulae to determine what the graph looks like of energy imparted to water (for sake of argument) in what proportion at what depth, but I won't get into that here. This may or may not be useful for some manner of non-supervillian related purpose, but I can't think of what that might be off the top of my head.

If the question is, "does exposure to microwaves cause any kind of freaky health effects, cause cancer, or induce mutant superpowers over and above just heating up the subject," the answer insofar as we can determine is no.

However, it is absolutely possible to cause a wide array of boring old regular injuries up to an including death by, not to put too fine a point on it, cooking people. It's very easy to bring water and other liquids to a boil by heating via microwave, for instance, and doing that inside the subject's skin is probably a recipe for disaster. You would have to be able to accurately control the amount of energy imparted on the subject, and you would need some compelling reason why you'd have to do that via microwave versus innumerable more traditional methods many of which are inherently fail-safe, like just immersing them in warm water.

Have you seen the Tom Scott video?

If it works for hamsters may be a properly built one could work for people, but there's absolutely no ethical way to test that.

Would you feel warmer if you were cooked? I'm not sure you'd feel much of anything, myself.

Infrared heaters already exist. They're used in spaces where you want to only heat people and not the air, like shops.

They're very effective. You know when you walk under them.

Keep us posted ....

I once did three weeks of IT work in a forensic pathology lab.