The problem is figuring out where to add it. Since the messages may be built with HTML, the link position can't simply be placed below all of the other code. It really has to be placed manually.
I could probably add an alert message warning you that you haven't added an unsubscribe link, but actually adding it wouldn't be possible.
If you want to give me a call I'll do screen sharing with you to help you add the error message so you don't forget moving forward.