Hi,
HikaSerial use the "onAfterOrderProductsListingDisplay" trigger (yes, it's a big name).
In the order_status_notification_html email you would found this :
<?php
JPluginHelper::importPlugin('hikashop');
$dispatcher =& JDispatcher::getInstance();
$dispatcher->trigger('onAfterOrderProductsListingDisplay', array(&$data->cart, 'email_notification_html'));
?>
You can add this trigger in your customized emails.
If there is no serial attached, HikaSerial would display nothing, otherwise it would use his view "orderserial / show_email_notification_html" as template.
Best regards,