Hello Dave,
we are considering allowing newsletters to be a plain text as a new feature for next versions.
The newsletters templates do not relate to e-mail templates in Site manager (they work e.g. for notification e-mails).The newsletters have their own templates in CMSDesk. It is not possible to use templates from Site manager for newsletters.
However, you could develop a custom module which would send e-mails according your needs. The sample code of sending e-mails with template by API:
How to send an email using the email template.
Best regards,
Helena Grulichova