You could create a Segmentation called "Opted In" with Segment=Opted In or Opted Out.
Then you'd create a Snippet or Dynamic Content for that Segment.
So this wouldn't be "hard coded" into the template, rather it would be an email that you would clone each time you run an email.
The problem I foresee is when you want to use a totally different Segmentation for the same promo email. But maybe that wouldn't actually happen in practice.
Why not just always have a Subscribe option? Those who subscribed will just ignore it. Or you could run specific Opt In to Our Newsletter campaigns. This might be better done on your blog/website than in promo emails.