I sum the speed squared voltage and the voltage proportional to current in an error amplifier and feed the error signal to the pwm. Perhaps this was what you had in mind, it is just a servo loop as used for any analogue feedback control.
This way the current is proportional to the reference signal.
Interesting things to watch for are that if the "speed squared" signal limits then the mill goes to constant current ( run away and avoid it). If the current signal limits then it aims for constant speed and that is nice to hold the things under control and induce a measure of stall at the furling point.
For those trying to follow this but not completely understanding, it is no way a mppt controller. It is a controller to force an alternator power out to follow speed cubed at the input. It has to be set up to match a given machine. The set up is not difficult, you have to set the pwm at cut in ( error amplifier offset) and set the current gain of the current amplifier to get the correct pwm at full load.
Flux