Thought I'd follow-up and see if anyone has any ideas regarding how to enable AirPlay 2 in apps on the Mac outside of the Music app (e.g., Finder).
I spoke with Apple support regarding this today — they bounced be back-and-fourth between the HomePod and the Mac teams with no resolution, although representatives from both teams said that this "should" be possible.
After spending >$600 on HomePods that are widely-advertised to work great as a stereo pair, it is incredibly frustrating that this isn't easier to implement.
Currently, my best guess is that there is a major bug in the macOS Catalina AirPlay 2 stack that prevents the system from recognizing paired HomePods.