I have my root htaccess setup so it permanently redirects non-www and/or no-trailing slash request to www with trailing slash. It doesn't work with Zenphoto or Wordpress installed in subdirs but Wordpress has it own way through the permalinks plugin.

Is there a way to implement such redirects in Zenphoto to avoid duplicate contents?

You should be able to modify Zenphoto's .htaccess file to manage such redirects. Zenphoto does not have a dedicated way to manage permalinks yet. I believe that is (or should be) planned for the 1.3 roadmap.