Nice!
Sorry I was away from this, but I started Node-Red and also using broadcasts.
I’m gonna mark it as a solution so people can directly go to your link.
I kinda highjacked your post anyway. What started as a few improvements on your solution became a bit bigger step by step 
Thanks!
Νο probs mate, happy that someone brought it to the next level
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Not to revive an old thread but another way to go about it is having snapcast have multiple streams where mpd is going to have lowest priority, bluetooth second and vlc (setup as a telnet reciever, where you’d send your tts or whatever noise you want to)
Like so
Snapserver.conf
…
source = pipe:///dev/shm/snapserver/snapfifo?name=Storm
source = pipe:///dev/shm/snapserver/bluefifo?name=Bluetooth
source = pipe:///dev/shm/snapserver/vlcfifo?name=VLC
stream = tcp://192.168.0.11?port=4953&name=Broadcast
source = meta:///VLC/Broadcast/Bluetooth/Storm?name=Mix
…
Start vlc like so on the machine running mpd/mopidy and snapserver(preferably as a service/daemon):
vlc -I telnet --telnet-pasword=yourpassword vlc --no-video --aout afile --audiofile-file /dev/shm/snapserver/vlcfifo
Sorry for the missing code tags, on my phone and can’t find the tags