We have a button for this in Appointment Reminder. If I recall correctly, the implementation pops a modal and says "Sure thing, we can get any email template you want set up for you. Send it to support@ and we'll get it ready within a day or so."

I've done this a grand total of once, so didn't spent 3 months building out the rest of the feature.

I use the same trick for doing data import. I'd love to get that working in an actually automated fashion, because it does get used pretty frequently, but unfortunately arbitrary column-to-column mapping UIs are hard to do well.