Magento 2 – Override Order Email Template

Var $payment_html is injected in email variables in Mage_Sales_Model_Order::sendNewOrderEmail() and not available for update emails. Since exact block wich is rendered depends on payment method you will need to get that HTML using same way as in Mage_Sales_Model_Order::sendNewOrderEmail(). You can either extend and overwrite Mage_Sales_Model_Order::sendNewOrderEmail() or create your own block and reference it in the email template.

{{block type="your/block_type"}}

The only problem is how to pass order to your block. I am not sure if var order will work in this context.

When passing a variable this way:

{{layout handle="sales_email_order_items" order=$order  test=$test}}

You make it available in the email/order/items.phtml template only.
You are able to get it there with: $this->getTest() or $this->getData('test').
To pass it further to the email/order/items/order/default.phtml edit the items.phtml file and add this

$item->setTest($this->getTest());

before calling

<?php echo $this->getItemHtml($_item) ?>

To make it shorter, replace:

<?php echo $this->getItemHtml($_item) ?>

with

<?php echo $this->getItemHtml($_item->setTest($this->getTest())) ?>