I don't beleive it's possible I'm afraid. I had a very similar setup to you and just used to set the 502 to a specific volume when watching HT stuff. At least it's easy to always set the 502 to the same volume as used when you configured the A/V amp.

System: DSP7200SE (upgraded) + RPI (PiCorePlayer) with Allo Digione running Squeezebox system.

Also various legacy 500 components - 557x2, 502, 508-20bit, 566-24bit plus too many other misc amps/speakers.