If it is the type where calibration varied with frequency, you could copy
what was done with some commercial instruments of the seventies, eg the
Oskerblok.

1. Calibrate the sensitivity knob with a logging scale.

2. Add a power scale to the meter face.

3. Measure a set of points of sensitivity setting vs frequency where the
meter scale is correctly calibrated for power.

4. Create an interpolated smooth graph as a lookup, and attach it to the
instrument.

Be prepared for significant linearity error at the low frequency end if
you calibrated the meter for low fsd power.

If it is a Bruene type sensor, the response should be reasonably flat
within a frequency range which you must establish.

The interesting thing with either type of sensor (and both are used in
Bird elements, though the first type is frequency compensated), is that
although the Pfwd and Pref power readings are of each of no value by
themselves, the power in the line is Pfwd-Pref irrespective of the load
impedance or line Zo.

Owen