Servoed PIN diodes? Of course then you'll have to find nice dual PIN diodes with the desired minimum carrier lifetime, low enough capacitance and such. If you find suitable ones you could do this:
One goes into the RF path. The other goes into a separate amp and a pilot signal of precise amplitude is injected. This feeds some chip that has a good RSSI output and there is your servo signal you can use to offset for drift in the PIN diodes.
Depending on how drifty your stuff is and thus how much calibration range is required you may need more than one pair.