This app note describes a simple, inexpensive constant power driver:
A combination current-sense amplifier and multiplier (MAX4211F) senses both the current through and voltage across the load, and supplies at one of its outputs (POUT) a voltage proportional to the continuous product of those variables, i.e., proportional to the load’s instantaneous power. External op amps generate a corresponding PWM (pulse width modulated) output signal that controls a p-channel MOSFET in series with the load.
Thanks rsdio!