How to do select facets in the fieldset realm?

I was trying to make some facets display as select boxes in the fieldset realm, but all of the select boxes were empty. In looking at this code from luceneapi_facet.module:

/**
 * Converts a facet to a FAPI array.
 *
 * @param $facet
 *   An array containing a facet retrieved from hook_luceneapi_facet().
 * @return
 *   A FAPI array.
 * @todo add some sanity checks
 */
function luceneapi_facet_to_fapi_convert($facet) {
  $base_form = array(
    '#type' => $facet['type'],
    '#title' => check_plain($facet['title']),
    '#weight' => $facet['weight'],
    '#description' => filter_xss($facet['description']),
  );
  switch ($facet['type']) {
    case 'checkbox':
    case 'date':
    case 'textarea':
    case 'textfield':
      $form[$facet['element']] = array_merge($base_form, array(
        '#default_value' => (string)luceneapi_facet_value_get($facet['element'], '')
      ));
      break;
    case 'checkboxes':
    case 'radios':
    case 'select':
      $form[$facet['element']] = array_merge($base_form, array(
        '#options' => array(),
        '#default_value' => (array)luceneapi_facet_value_get($facet['element'], array())
      ));
      break;
    case 'markup':
      $form[$facet['element']] = array_merge($base_form, array(
        '#value' => $base_form['#title']
      ));
      break;
    default:
      $form = array();
      break;
  }
  return $form;
}

I can't see how the #options can get set, other than doing a form alter. Is this intended behavior, or am I missing something?