Hi,
You're limited by the interface to generate the tags in the newsletter. It's done by the line:
$values = array('product_id' => 'ACY_ID', 'product_created' => 'CREATED_DATE', 'product_modified' => 'MODIFIED_DATE', 'product_name' => 'HIKA_TITLE');
in plugins/acymailing/hikashop/hikashop.php
So you could add your custom field in that array to be able to select it for your tags.
But even without that, you can just replace the value of the order parameter in the tag in the newsletter.
Change the piece:
order:product_id,DESC
to for example:
order:XXX,ASC
where XXX is the column name of a custom field of the table "product" and you'll be able to sort the products of that tag based on that custom field, even without any modification to the code.