Hey
can't find any mention of this already, although someone must have though about this already! - anyway here's my thoughts:
Would be nice if inside the main webapp I could mark tracks/albums/playlists to be available offline on my mobile, similar to spotify (never used this in spotify so not 100% sure how it works)
Then create a sync api call that tells clients what tracks to to sync. The client can then use the track id list returned by this call to download the tracks and store locally.
The client apps would obviously have to update to support this. When they start up they could have a button somewhere that puts them in "offline" mode perhaps? Then any browsing would only be on the local files.
Many thanks for subsonic - it's a good bit of software that seems to be getting even better!