[ori_b]

This is a fundamental problem with standing waves in microwave ovens, and isn't really affected by pwm. You need to change the mean free path.

[tpmx]

I've never tried a Panasonic inverter-style microwave oven, but I imagine it would solve this problem. It would be able to run at like say 30% for 30s while rotating the contents.

[sitharus]

I have a Panasonic inverter microwave, and it does solve the problem quite well. The standing waves aren't solved, but the lower continual heating lets the energy conduct in the food without overheating parts.