Look in the tinymce_plugins/superemotions/ folder at the source code there. It’s basically a custom tinymce plugin that uses WordPress shortcodes to display the WordPress emoticon images.
You can go a lot further than that. Also remember that WP Super Edit hooks into the way WordPress initializes the editor. It doesn’t replace the core editor code. Any WordPress plugin can be written to create custom editor buttons, and WP Super Edit “tries :)” not to interfere with other custom visual editor enhancements.
I do need to start working on WP Super Edit again. I think the database tables are already set up so that a completely separate WordPress plugin can hook into WP Super Edit, but I think I need to make sure that I take that a couple steps further. Yea, and fix some bugs.
Oh time keeps on slipping.. slipping.. slipping.. Into the future.