This is hook_form_alter from signature module:

//cleaning signature in comment/reply
  if ($form_id == 'comment_form') {
    $form_values = _comment_form_submit($form_values);
    if ($cid = comment_save($form_values)) {
      return array('node/'. $form_values['nid'], "page=".comment_last_page($form_values['nid']), "comment-$cid");
    }

    global $user;
    if ($form['comment_filter']['comment']['#default_value'] == $user->signature ) {
      $form['comment_filter']['comment']['#default_value'] = '';      
  	}
  }
}


I want to sneak this into it: