I have heard that microwave turntables absorb microwaves when there is very little load in the cavity, which limits the reflection to the magnetron. Will this also limit the leakage radiation from the cavity? Also, would similar sized plates be interchangeable in regard to the microwave absorption?
Microwave turntables are in most cases made out of glass. As the food can heat up considerably it is likely that more heat resistant types such as borosilicate glasses are used.
Regardless whether the microwave is loaded with food items or empty the glass will absorb some radiation which heats up the glass turntable. This absorption at temperatures below 400°C is very weak. In an experiment a microwave power of over 1 kW and 10 minutes of exposure was required to heat up SiO$_2$ to such high temperatures. So for lower powers and shorter times the glass from the turntable does not strongly influence the radiative properties of the microwave.