Ah sorry, forgot to say it's mpd with http streaming. Adding music is done via scp in my case, but on another server (music library is mounted via NFS)

Accessing the NFS share could be done on the VPN as well, this way users could upload and fully control the radio when connected

Something nice could be to use a FS distributed between users. When connected, their music folder would be mounted automatically (using something like overlayfs, but over the network), amd added to the playlist (which would play random songs)