Thanks Frank, I tried out Airfoil now. Does the job. However, I'm investigating also another option with using zones.
On my PC I have TuneAero installed which serves as an Airplay receiver. I can see this "device" from within MC on my MacBook as an output device BUT only if it is selected as an output device in macOS sound settings. As soon as I change the sound device in macOS to "internal loudspeaker" or something else than TuneAero, the corresponding output device in MC disappears. This is 100% reproducible since the output device appears in MC as soon as I again select it in the macOS settings (options in MC need to be closed and reopened in order to refresh the output devices). If the TuneAero device would stay available in MC as an output device, one would be able to use it as output in another Zone. This would be a very elegant solution to this use case since then one wouldn't need a streamer app (e.g. Airfoil) anymore.
So: can the TuneAero device be made persistent, regardless of sound device settings in macOS?