This is contorted and I don't have anything handy in order to try, but one possibility is to loop a Zone2 or Zone3 analog output back to an analog input and reassign audio for the HDMI input to the looped back analog input. This can be done through the receiver's setup web interface or a control system. Offhand I don't recall if an RTI or URC driver can get down to this level.

Another approach would be to split the HDMI and make two connections to the receiver -- one using the regular HDMI audio and the other using my loop back scheme.