Okay, my HDMI dongle arrived yesterday, so this afternoon was the initial session to try CEC/ARC etc.

First problem: ARC was unreliable. It would work in about 8 second bursts.. turned out to be a cabling issue. I replaced the skinny HDMI cable with a pair of thicker ones and a coupler to join them for the needed length. Problem solved.

Then I inserted the HDMI/CEC dongle in place of the coupler, and with a small amount of scripting and an hour or so of time.. I got the MythTV box to control the AVR's volume/mute over CEC. Yay!

Power on/off and input switching also seem to work with simple commands over the CEC tap (USB interface to MythTV box).

So I'm back to a single remote control for everything here again. Not that that helps Andy.