Well, while inotify would be overkill (imho) (and not being supported on must nslu2 flavours), a simple
for $file in *.[mp3][ogg][flac];do foo;-esque script would do. where foo does an sqlite insert in the mt-daapd style. trigger that every 5 minutes, and have a dir structure like this:
incoming/
new music/put the new stuff in incoming/, the script iterates every say, 5 minutes through that directory, copies the files to new music/ and then adds it from that path. voila.