In order to try and keep the diffracted power at a nominal 8%, I tried a simple ezcaservo to do this.
Running the following on opsws2
ezcaservo -r "H1:PSL-ISS_DIFFRACTION_AVG" -s 8.0 -g -0.001 -f 0.05 -t 240 "H1:PSL=ISS_REFSIGNAL"
This timed out after 4 minutes.

Increasing the gain to -0.01 seems okay.  -0.1 is too high. -0.05 is too high.  -0.02 seems okay.

The attached plot shows two manually introduced excursions from the reference signal set point and
the recovery done by ezcaservo, which seems to work okay.  The final command issued was
ezcaservo -r "H1:PSL-ISS_DIFFRACTION_AVG" -s 8.0 -g -0.02 -f 0.05 -t 240 "H1:PSL-ISS_REFSIGNAL"

The command timed out and is no longer running.  It would be worth taking out for a longer test
drive at a later time.