no way to thematize comments as a whole

I'd like to place all the comments in a collapsable div, but there is no way to do this other than to modify comments.php:comment_render and add:

  $form['commentwrap'] = array('#type' => 'fieldset', '#title' => t('Comments'),
                                 '#collapsible' => TRUE, '#collapsed' => TRUE);
  $form['commentwrap']['comments'] = array('#type' => 'markup', '#value' => $output);
  return drupal_get_form('commentwrap', $form);

It'd be nice if either here or in node.php:node_body which calls comment_render, there'd be a place for someone to use the theme engine to change the look and feel of comments as a whole, rather than individual comments.