Good news, bad news situation.
Of all the things I'm not very well versed in; child devices.
I am sure I can learn it fairly easily but it requires first hand testing.
My S720 was a starter unit. Wanted to see what Denon really had to offer over Pioneer.
I have been searching for something with multiple zones and pre-outs. It's just a matter of time until I get something.
If I can duplicate it for zone 2, it's going to be easy to write it for 3.
If you need immediate remedy, I can get your instant message address and we can work together on it. The first would be getting the code for zone 2 on your Hubitat and seeing if it works at all.
A lot of things available on the child device are not supposed to be available (which I saw on the word doc posted above). To that end, I could at least make sure those are taken out.
Groovy is a very easy language to understand so, it just takes reading it through to figure out how each variable is assigned.
Let me know.