Its not that hard.
You just need some redirection scripts.
If your music library is mounted upon the nslu2 startup and accesible, then a 1 line script would suffize.
check out
http://www.nslu2-linux.org/wiki/Unslung/DiversionScripts
a script would look like this:
#! /bin/sh
/opt/etc/init.d/S60mt-daapd
return 1
sort of. If your music stuff isnt mounted, you can take care of the mount process here as well. And do various other stuff.