I’m a greybeard chartered engineer, as you may have noticed from the ability to talk definitive nonsense! 
The generic optocoupler modules arrived from eBay in the post this morning, so after some minor fettling, ESPhome is logging the heater state to get an idea of typical heating control duty cycles.
As a taster, here’s a log for Level 2:
As you can see, the pattern isn’t quite symmetrical, but the duty cycle is regular suggesting there’s no magic to controlling a towel radiator heater.
More to come…