Thanks for looking at this. Using libavahi from the armv7 repo did indeed work, but I've now done a clean install and tce-load -wi cups now completes without any error messages.
I haven't checked whether avahi actually works, but I have got cups working. There was initially a problem that once the cups service was started, its web configuration interface didn't work properly: you could connect from other machines, but it either gave 404 "Not found" errors, or stylesheets and other resources were missing from the web pages. Looking at the logs, this seemed to be because it objected to these resource files being symlinks. I fixed the issue by adding the following to my /opt/bootlocal.sh file, to create the symlinks at a higher level in the directory structure at boot time:
sudo rm -rf /usr/local/share/doc/cups
sudo ln -s /tmp/tcloop/cups/usr/local/share/doc/cups /usr/local/share/doc/cups
sudo /usr/local/etc/init.d/cups start