Given the interface where newsletter are sent from the New Topic interface, it seems very natural to have it posted as a new topic. Of course, this is quite easy currently, you can just "Send As Newsletter" and then click a second time and Post it. But if the plugin handles it, the advantage is that the Reply-To address on the mailed newsletter can be the correct one for replying to the topic, and the body of the newsletter can contain a link to the topic on the board as well.
Let me know if you think this is a good idea; I'm willing to work on it.