The playlists are stored in SQL.
Reloading playlists from files will be a bit tricky to do. The other way, syncing playlists to disk would be easier and makes more sense.
An option for this could be added to the playlist settings. This needs to be done in the Playlist Backend: https://github.com/strawberrymusicplayer/strawberry/blob/master/src/playlist/playlistbackend.cpp
You can see how playlists are saved to file in the Playlist Manager here:
An easier solution would be that you just create a script that retrieves the playlists from SQL and saves to them to files.