Other HTML allowed in emails

Hi code people,

I have just discovered this brilliant plugin (congrats, it’s VERY useful) and wanted to send a custom email with a “donate” button in it, on a “Pay as you feel” product. The ideal button is in a PayPal form which cannot easily be done through a link.

Reading previous posts here (and understanding code), I edited admin/class-custom-woocommerce-field.php and made this addition:

                   $allowed_html = array(
                       'form' => array(
                                'action' => array(),
                                'method' => array(),
                                'style' => array(),
                                'target' => array()
                                ),
                        'button' => array(
                                'name' => array(),
                                'type' => array(),
                                'value' => array()
                                ),
                        'input' => array(
                                'name' => array(),
                                'type' => array(),
                                'value' => array()
                                ),
...

This is great and works as I need, but if there is an update to the plugin it will disappear. Might there be a way to customise the allowed html in a file that persists after such an upgrade?

Cheers
LymeSimon