as I posted on Computer Audiophile....
...sigh.... I saw that thread and it made me sad
I haven't had a chance to try out Tidal and Roon yet so I can't comment on their desktop or tablet UI but after a few days of Spotify I could see spending some (lots of) time grazing in their music library and being quite happy with their Android UI.
...
You should be able to set up another zone with your USB DAC and use the MC WDM virtual sound card driver as the sound device for Spotify.
There's actually a few ways to do this.
I already have Spotify and other streaming services routed through my whole house and interchange between them seamlessly from my phone or tablet.
I use the native iPhone or Android App to control the streaming players and MC WDM sound card to output the MC DSP to my sound card.
The other streaming applications just sit in the background and MC Zone switcher has some rules to automatically route the audio to the right place depending on what it is. I haven't got time to go into detail right now but it should work for what you want to do.
You will possibly have to use the USB DAC in shared mode in the loopback zone as opposed to exclusive mode to enable seamless switching but I definitely have it working that way! (and with the TuneBlade and TuneAero to simulcast around the house)
If you're not interested in synchronized audio around the house it's really just as simple as setting the MC WDM virtual sound card up as the streaming default device in a dedicated zone with some rules.
The only thing you wont get is UI integration with MC, but I either have the screen turned off and use the native tablet app or full screen the native App on the PC and MC goes to the background.