If you are putting in any other automation, doing this with any controllable receiver would be fairly trivial, as long as the automation system supports Sonos of course. For CQC you'd just set up a triggered event on the Sonos driver's media state field. If it goes to playing, power up the receiver and select the correct input. We could even only do it if a certain station was playing as well, or only if particular Sonos players start playing, ignoring others.

I do kind of question how really appropriate such auto-magical things are, since it may not always be appropriate and you can't easily stop it from happening.