I noticed that Chromecast Beta app got updated on December 6th and I went to go check if I could add one of my official Android TV's to a Google Home Speaker Group and......
I could add my Nvidia Shield 2019 (Tube version) to the speaker group in my Google Home app!!!
Nvidia Shield 2017 or Mi Box S however could not be added. Anybody else add their Nvidia Shield 2019 (Pro or regular)?
Working on my 2019 Sony Bravia Android TV.
Bit glitchy, but working.
Edit: Audio is a fraction of a second of so gives an echo effect. Could be my network.