I wonder if a good solution would be to have a fairly hefty capacitor taking charge from the supply side and then have the system measure the capacitance to determine how it will light the bulb from there. It could check for a capacitance drop to know it is using too much power and drop the output brightness using voltage or PWM, then bring it back up as the capacitor fills more. You would definitely get fluctuations, but increasing the capacitor size would absorb most types of fluctuation and also allow a system agnositc approach, able to ignore whether it is a PWM, sine cutting, or other method of dimming on the power supply. A nice benefit would be the very slight curve in the light output when you turn it off, just extending the lighting a few parts of a second as the capacitor drains.