The variables array generated here is a mirror of template_process_html(). This processor will run its course when theme_maintenance_page() is invoked.
function template_process_maintenance_page(&$variables) {
$variables['head'] = drupal_get_html_head();
$variables['css'] = drupal_add_css();
$variables['styles'] = drupal_get_css();
$variables['scripts'] = drupal_get_js();
}