// ... $form_state and $form_info all populated now.

  if ($js) {
    ctools_include('ajax');
    ctools_include('modal');

    // Neither of these 'ajax' or 'modal' seemed to have any affect.
    $form_state['ajax'] = TRUE;
    $form_state['modal'] = TRUE;
    $form_info['ajax'] = TRUE;
    $form_info['modal'] = TRUE;

    // This seems to work.
    $form_state['ajax render'] = 'my_module_ajax_render';

    $output = ctools_wizard_multistep_form($form_info, $step, $form_state);

    if (empty($output)) {
      drupal_set_message('All done.');
      $output[] = ctools_modal_command_dismiss();
    }

    // JSON object is returned while ajax render is set as above.
    return $output;
  }
  else {
    // Outputs wizard just fine.
    return ctools_wizard_multistep_form($form_info, $step, $form_state);
  }
}


function my_module_ajax_render(&$form_state, $output) {
  $messages = theme_status_messages();
  $commands[] = ajax_command_replace("#modal-content", $output);
  return ajax_render($commands);
}