first you would go ahead and install these:
http://ipkgfind.nslu2-linux.org/details.php?package=alac-decoder&official=&format=
then you have to add .alac (or whatever the extension) to the ssc-codectypes in the mt-daapd.conf file.
Finally, modify the /opt/sbin/mt-daapd-ssc.sh script so it really does transcode the ALAC files to WAV.
Just copy the parts from other codecs, but be sure to use the right decoder binary.
As far as realtime is concerned, Im not able to advise.