Display custom text when comments are closed

If you go to comments.php in your theme – and search for “Comments are closed”, you should find it.

You should also be able do this in functions.php:

function comment_text ($arg) {
$arg['title_reply'] = __('Too Late - Comments are Closed!');
return $arg;
}
add_filter('comment_form_defaults','comment_text');

Edit: Just noticed that you want this to work cross theme. So if you go to: wp-comments-post.php in the top level of your WordPress install, you should find it there too.