If you look at the builtin WordPress widgets, they apply widget_title filters against the title. Is it possible to do the same for the Top Commentators plugin?
The 'widget_title' filter is used in all the builtin wordpress widgets (wp-includes/default-widgets.php).
It's a small change. apply_filters needs to be added to widget():
$title = $instance['title'];
$title = apply_filters('widget_title', $title);