The way I've noted that _should_ work (I haven't made it fully work yet).
Pulseaudio allows you to stream data out via DNLA and Airplay, both of which the MDs support, to an extent.
I've had music from desktop spotify play via an airplay stream to an MD, although no, actual, sound came out, both ends recognised each other and connected to each other.
I haven't investigated further, but that might be a possibility.
The other one would be to integrate pulse audio into an MD as another audio server option alongside an airplay server, as pulse audio network playing is pretty solid in my experience.
it can do WHAT!!?>!>!>
sometimes, i lament being in Qt / QML world day an night. So many other technologies and so little time...