I have had a go at adding an extra plugin from the list of available TinyMCE plugins to WP Super Edit. In this case the simple horizontal rule (hr). It seems to work but I am interested to know if I have done it the right way as I can't find any documentation on how to do it.
I added a line directly into the wp_super_edit_buttons table in the database:
nicename: Horizontal Rule
provider: wp_super_edit (though "wordpress" works as well)
I did not upload any files.
The drawback I suppose is that the new plugin would get wiped from the database on resetting and perhaps even on updating WP Super Edit?
I know you can easily add TinyMCE plugins to WordPress using the functions.php file in a theme but I want to use the role-based functionality of WP Super Edit which seems to set it apart from other, similar plugins.