Posted too soon :-(
It appears IPFW may not be able to filter AFP (file sharing) after all. Worth a try possibly, but may not work.

Try using the advice in this tip: http://www.macosxhints.com/article.php?story=20080119112509736 which demonstrates bandwidth throttling by port number
but add a rule that limits by ip address as well as port number
see http://developer.apple.com/mac/library/documentation/Darwin/Reference/ManPages/man8/ipfw.8.html for details of the ipfw rules
I haven't tried this combination myself but I can't see why it wouldn't work.