Today’s update to iOS 11 includes new controls for audio playback on external devices, as shown in the screenshots here on MacStories:
Besides adding compatibility with HomePod, the primary user-facing feature of today’s release is a new set of controls for audio playback on external devices. When viewing Control Center on your iPhone or iPad, if you open the expanded audio playback tile (either by tapping the signal icon in the top-right corner, or by using 3D Touch or a long press), compatible external audio sources now display as separate UI tiles underneath the main audio tile. As seen above, the Apple TV is a supported audio device. Opening one of the additional audio tiles allows you to control playback on an external device while having separate playback controls from what’s playing on your iOS device. In my testing, I could set an album in Apple Music to play on my Apple TV while listening to a podcast in Apple Podcasts on my iPhone.
Would it be possible to investigate using these Control Center readouts and/or controls for Roon? I imagine this might work, at the very least, for AirPlay and AirPlay 2 endpoints.