Yup that's exactly the idea, for it to approach equilibrium. And the microwave should be able to deduce how close it is by the rate of cooling of non-hotspots

Yup that's exactly the idea, for it to approach equilibrium. And the microwave should be able to deduce how close it is by the rate of cooling of non-hotspots while the cycle is off. If it's cooling quickly, the inside is still frozen. If it isn't cooling at all, the inside is nice and warm.