Peoples thoughts and interest in the project
I've always had in mind the idea of an interface that could be used with different roasters whereas this would be very much Hot Top specific
You making a USB controller for a roaster - yes, very interesting. For the HotTop per se - well... I think it might be even more interesting with a general roast profiler, something that could be used for many different roaster types.
Also, I might add a slight preference for the PWM method, as there is quite a bit of effect to dim here. Besides the problem of obtaining high effect dimmers (with triacs and such) that can switch >1.5 kW, modulating the sine wave like dimmers do (by cutting it off) is known to pollute the grid, sending the switching noise back into the wall outlet and to surroundings. Radios nearby, and often amplifiers too, pick it up. Don't like it.
On the other hand, a "slow" PWM circuit, with, say, a 0.5 or 1 second base period, should get rid of all that potential noise, even at high effects. I have such a controller at home, haven't used it for anything yet but it is a very simple looking control. Looks and function of a smallish potentiometer, turn the knob and you get more effect through it. Only it switches up to 2.5 kW without a problem, 1 second intervals. You would need something without a knob that needs to be turned to the right position though.
I will follow this closely, no matter which road you choose. Most interesting!