I would recommend looking into newsletter platforms that have a Magento integration. I'm not sure they can do exactly what you want, but they can make your life a lot easier. There are several like Bronto.com or Magemail.co that have different features that are targeted for various markets and business sizes.
Otherwise, the HTML you need for your newsletter is going to be different than anyone else's based on your newsletter template. It should be a fairly simple module to create that has an admin page where you can pass a product id and it output only that html in a text area with a copy button. Then, you can have it add a link on the product edit page so anyone can click it, get the html, click copy and then you are ready to paste into your newsletter.