I have used custom HTML Templates with custom fields, however they used the OOB notification forms with the custom fields added.
The templates had embedded links to the Portals i.e. DWP and SmartIT.
Is this the kind of thing you are after?
I have added custom fields to the SYS:Notification Message so I know how to do that. The issue is the new Approval Process that is OOTB that DEMP uses. There is no specific template.
you can either embed the HTML into the message body (as done in the Approval via email messages), or attach an existing Template (including any custom ones) to the SYS:Notification Messages entry.