Microwave frequencies used in ovens would be dangerous because they can penetrate deep in the body and heat internal tissues, but in the EHF range microwave heating could be safe. See for instance https://en.wikipedia.org/wiki/Active_Denial_System
With all that said, what I had in mind is in the far less scary and far more prosaic near-infrared range.
One concept I've toyed with for a while is a laser blanket. A ceiling mounted infrared camera measures the temperature across your body, and an infrared laser automatically goes and "paints" any part that is insufficiently warm. No idea if it would be comfortable at all, but fun project.
Microwave frequencies used in ovens would be dangerous because they can penetrate deep in the body and heat internal tissues, but in the EHF range microwave heating could be safe. See for instance https://en.wikipedia.org/wiki/Active_Denial_System
With all that said, what I had in mind is in the far less scary and far more prosaic near-infrared range.