Back to the subject of derating furnaces...
If modulating furnaces only change the inducer blower speed and the gas manifold pressure, I don't see why I couldn't wire a dimmer switch inline with the inducer motor and adjust the manifold pressure on the regulator. The inducer motor seems to be a shaded-pole AC motor, so a dimmer switch should be sufficient to control the motor speed. I would want to use a more sturdy dimmer, not just an off-the-shelf cheapo. I have used those before for this purpose and they don't maintain motor speed very well.
The fuel-air mixture could be adjusted with reference to the exhaust temperature and temperature rise over the heat exchanger.