Light Timer behind a dimmer?
I would use a simple intermatic timer being powered by another source then use the contact set in the timer to break the power to the heater. This would be a code compliant way to control the heat cycle. I do not know of any timer that could work as you describe but they may be out there. As far as intermatic they make clockwork style that you put lugs on a dial up to fancy digital units that you program. There are other brands I use intermatic because they last at least the clockwork ones almost never fail.