I was thinking it might be nice to add some insulation around some the pi's enclosure, to reduce its cooling significantly. A little bit to tamp down any potential rapid fluctuations in the room's temperature (if someone opens a window, steps out of the bath, whatever). But more so that it could save a watt or two of power, by having the time-burner cores working much less.

You're right that this is a over-controller oscillator. The goal generally with ovens is to keep heat! (To an extent of course.)