I know it's been a while, but in my quest to figure something similar, I found your post.
I have a different setting, but close enough, that I wonder if you ever found a solution.
W7 / Asrock z77 Extreme4 / i5 - using onboard HDMI and graphics -> AVR (HDMI)
AVR HDMI Output 1 -> HDTV
AVR is setup with HDMI standby passthorugh
If I leave the AVR off, w7 recognizes the AVR fine.
If I power on the AVR, w7 shows the HDTV instead, and there's no AVR in the list.