I use signature function a lot , it creates consistency and saves a heap of typing.
This would be either part of that, or and addition, like a banner add. Preferably one that rotates. So in a conversation email a different ad would appear. Could be as much as a single line of text to a full on banner.
So thats the starting point comments, ideas, smackdown!
If you already know of a product please let me know save reinventing the wheel