Well, despite no help on this question, after hours of research, I figured out how to do this. I'll post here in hopes it helps prevent someone else from having to spend hours researching.
I came across this list of endpoints for Denon receivers:
You just have to put " http://xxx.xxx.xx.xx/" before each command where the x's are the IP address of your Denon receiver. You'll also want to make sure your receiver is on a reserved or fixed/static IP address on your network. On my Google Wifi, I just have to tell it to reserve an IP address for that device so it doesn't change rendering the commands useless.
Then I programmed a Lutron Pico Remote, using @stephack's Advanced Button Controller app, so that a quick press of button 3 turns on zone 2, a long press turns off zone 2. Button 2 increases volume and button 4 decreases volume. These were each done using the "Send Http Request" feature. I found using the "Get" command rather than the "Post" command is much faster. The commands sent using "Get" are almost instant whereas commands sent using "Post" have a noticeable delay which is frustrating especially when trying to adjust volume. I have no clue what the difference between "get" and "post" are...
But now I can be on our patio, tell Google to play whatever music from Spotify on the Denon Receiver, then hit a button the Pico remote and have the music playing out on the patio with the ability to control volume.
There are a few improvements I would like to try and implement if possible:
- ability to have a button press press move the volume up or down more than just one notch as this is such a minor adjustment, often need to adjust by 5 notches at at time
- similarly, would be nice to have a button hold ramp up or down volume
- ability to just say "hey google, play whatever music on patio" and have it start playing the requested music on the receiver and turn on zone 2 all in one command. Might have to settle with a double google home command: "play whatever music on Denon Receiver AND turn on zone 2"
