I did this a bit differently. First sorted categories with the My Category Order plugin, then added plugin “Drop Down Manager”
Edited wp-content/plugins/dropdown-manager/dropdown_manager.php line 55 to become this:
WHERE dropdown.menu = ‘” . (int)$num . “‘ order by terms.term_order ASC;”;
Very easy for sorting drop downs but other php files/code would have to be edited for other uses.
