I got the following to do what I needed:
add_filter('the_editor', 'my_function');
Your function will be passed the HTML code for the on-page comment editor. You’ll need to be sure to return that code, otherwise your editor will disappear. Look in /wp-includes/general-template.php for more.
Sample code:
function my_function($html) {
$output = $html;
$output .= ...your html code if applicable
...or do whatever you need
return $output;
}
