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"