I've quickly checked the controller specs, and I do not think that it can as low as 100 Hz for the HW PWM output, which either leaves me two possibilities (I think):
Soft PWM using a triac (Do I need a snubber circuit of some sort? I've never measured the cos phi of a standard "commerce" 220V heater)
Using an IGBT connected to the HW PWM output, cycling at perhaps 1kHz. However, I fear that I need to install RFI filters on the mains side to avoid pumping the house mains full of harmonics, and probably some filter on the heater side to avoid having the harmonics break the insulation (as happens in "unprepared" motors when using a frequency converter).
Am I missing something here?