Did you ever manage to get this working?
It seems that the mailer strips out any tags so one is not able to provide the data within the email that Trustpilot can read.
Unfortunately, I don't have the time to dive into it to figure out how to defeat it in both mail interfaces.
Yes, I did manage to get it working. I added the code below to the invoice.tpl file. (/design/themes/responsive/mail/templates/addons/my_changes/overrides/orders/invoice.tpl)
This site uses the old way of file templates rather than the newer admin controlled templates.
Hope this helps
{scripts}
{/scripts}
Is that really having the tag outside the template block named "scripts"?