Single quadrant analog multiplier
As others have said you may get an unstable situation so, temporarily forget about the multiplier and check to see that you can control the voltage output of the power supply based on voltage feedback directly to the op-amp. Use your pot to check that you can control it across the levels you might need with the load resistances you might use.

For a simple and fairly accurate multiplier you could turn the current feedback signal into a PWM signal - duty cycle proportional to output current. Then, use the PWM signal and a couple of analogue switches to chop up the voltage feedback signal. Use a filter on the output to give you a dc value proportional to power. Try and use a fairly high PWM frequency so that the extra low pass filter can run at tens of kHz.

Linear tech make the LTC6992 that would suit your needs: -



It takes a dc input voltage (aka current reference) and converts it to a duty cycle of up to 1 MHz frequency. Here's the idea applied to a triangle wave and note that this is a 4 quadrant multiplier but who cares - it operates in the first quadrant just as well as the other three: -



If the duty cycle were more like 99% the resulting red triangle wave would be much fuller in amplitude and just fractionally less than the original triangle wave.

Finally the block diagram would look like this: -