Leave the physical file-i/o out of the equation and its still a nice feat.
There have been some scripts hacked together that filled the DB with metadata for songs in a /incoming directory, or similiar.
So I guess functionality to add a distinct file that does not reside within the mp3_dir should be doable, possibly via a XML request.
Then the actual DB insert could be handled by mt-daapd.
This could be neatly combined with a file upload mechanism through the webinterface at a later point, if necessary.