The problem is not data representation. Yes, you could build fancy display features into an app that understands email, and define a data format for representing those features in email attachments/parts. They could then display just fine in your Onlinepersona app. (Or you could just use HTML email, which already has partial support in some user agents, though it is not universal.) You could even go so far as to define a reply protocol for your app to share data edits via email attachments. Those replies would be useful to other people on a mailing list who run your app.

But at that point, what you're using is not a mailing list. It's an Onlinepersona app that happens to use a mailing list as a transport for your overlay protocol. To everyone on the list who doesn't use your app, its traffic would be noise.

In other words, the problem is not data representation, but adoption. Good luck getting all the world's email software to support your niche extensions. I think the most you can realistically hope for is to convince the members of your favorite mailing lists to either use your app or tolerate the noise it generates.

If you're confident that your app is wanted by enough people to make its development worthwhile, then go for it. Just realise that it won't be an email client; it will be an Onlinepersona client.