Dynamic content in subscriber confirmation email

Is it possible to have the HTML and text components of the confirmation email contain dynamic text fed from an outside source?

We have a back end service that auto-subscribes users to a "new user" list, and then sends them a confirmation email. We'd LIKE to have a coupon code in that email ideally. Or worst case, a link to a site we host but we want to make sure people aren't getting a coupon code without being part of the "new user" list (Each user gets a unique code) without asking them to enter in an email address (So having the subscribers email in the URL in the confirmation message would work).

I've been reading the API docs and it appears this sort of stuff is possible with the email templates (external data with the emails) but since the confirmation email appears to be a static HTML... I'm wondering if this is at all possible?

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