Hi Paul,
Yes it is described in part 4 of the link I posted above.
http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm
It seems to be what the pro I talked to was suggesting too. ie separating out the instrument response (which is presumed stable so measured infrequently) and extinction (measured on the night and adjusted for air mass using the AOD)
Cheers
Robin