Thanks for the detailed and helpful reply, THOR32. I very much look forward to seeing your post on the MOSFET power switch since I've been noodling around methods for the PWM-compatible switching myself, but you clearly have thought about this longer and more deeply than I have.
Perhaps you could take a look at the link I provided earlier (MOS Trigger Switch Driver Module FET PWM Regulator High Power Electronic Switch Control Board  to determine if it would work as a PWM-compatible switch for our application, especially since it's pretty cheap ($2.94). My main doubt about this device is that its control inputs are "signal PWM+" and "signal GND". The open circuit operation (for off) of the AUX outputs gives me pause for our application, since floating input conditions may not be accounted for in the device's design.
I think ESU would LIKE for us to use their smoke generators using the specialized outputs/input on the L and XL, so your reverse engineering is very valuable for simpler smoke generators.
I may do a little reverse engineering of my own of an ESU smoke generator with temperature sensor to determine how it's producing the "temperature" output, be it a thermocouple, resistance temperature detector (RTD), etc. It might even be a good idea to put a simple, low-cost temperature sensor on simpler smoke generators anyway. Of course, calibration is the bear and may not be worth the trouble.
Thanks again for your original post and follow-up answers.