What's wrong with having a setpoint of 0.16V? Many op-amps have an offset voltage of less than 1mV; with this offset, you can still achieve better than 1% accuracy at 6mW. It's simply a matter of ensuring your op-amp's inputs and the multiplier's ouput include 0V.
If that still doesn't work for you, you might consider using a divider. The input signal (power) can be divided by the measured output current (P/I produces voltage), and this signal can then be fed to the op-amp.