Why would a (working) theme function defined in nodequeue module not be registered in theme registry? Trying to overide a  theme function defined in nodequeue  from another custom module:

/**
 * Implements hook_theme().
 */
function nodequeue_theme() {
  return array(
       'nodequeue_arrange_subqueue_form_table' =&gt; array(
      'render element' =&gt; 'form',
    ),
  );
}


/**
 * Theme the subqueue overview as a sortable list.
 *
 * @ingroup themeable
 */
function theme_nodequeue_arrange_subqueue_form_table($variables) {
  $form = $variables['form'];
  // Build output snipped for clarity...
  $output = 'bunch of code';

 return $output;
}


I was expecting to use theme_registry_alter but upon doing a var_dump of $theme_registry in the alter hook (after cache was flushed), it seems the function is not registered. Did I miss something?
Try 

function yourtheme_nodequeue_arrange_subqueue_form_table($variables) {}


I don't think you can override module functions from the theme layer without theme_hook_function or hook_function_alter... You'll need to create a module to override nodequeue_theme() - take at look at drupal_alter()