Through my Odoo account manager I was able to get some more information. Below is the summary I submitted to the Support team and thought I'd post here too in case it helps others...
I was able to find the Views that control this email:
header and footer via: mail_notification_light
body via: sign_template_mail_request
This unblocked me enough to achieve my goals by creating inherited views to change the email. However I think it should be on the roadmap to implement this email using the same Template approach that most of the other emails use. This way less technical users can manage the template content and formatting.