I just wanted to let you know that while I am aware that this plugin does not currently support the 'Categories' post type, it kind of works.
For instance, when I go to edit one of my Shopp categories the WordPress SEO meta boxes show up and they even save the data that I input into them. However, that data does not display on the actual category pages themselves.
Just wanted to point this out to you for future versions of this plugin (if you intend to make any). Seems to me that if the meta boxes show up and save the data, that displaying it would probably be quite easy to do. I'm no PHP programmer or else I'd tackle it myself.