I have done this by using a custom widget, here's an example.
In your theme functions.php add
'name' => 'custom',
'id' => 'custom',
'before_widget' => '',
'after_widget' => '',
'before_title' => '',
'after_title' => ''
Then in your comments.php ( or whatever you want to put the widget ) call the custom widget using:
<?php dynamic_sidebar("custom"); ?>
Don't forget to drop the widget in the new area "custom" in your widgets panels, also remember than the widget can be called only once, so you either call it on your sidebar or your comments form.
Would be nice if you could duplicate the widget though, hopefully for the next version :)
You can see it in action here: http://www.doble-h.com