Let’s just say the script is an obvious prelude to a more robust plugin solution. It would definitely require at least basic email capabilities for that to happen, though. (Just listing email addresses? Bo-ring.)
I don’t see a way to provide users/authors to ability to, on their own, be excluded from the list, unless you have them enter something in say the user profile or an unused field (like say ICQ) that the script looks for. Ugly, though. The more robust user functionality 1.6 will offer (with it’s users_meta table) should allow for something like this to added quite easily. Especially as a plugin. But then I’d note at this point you’re setting yourself up for a “subscribe to list” solution, and I’d naturally have to suggest looking at what may already be out there, such as:
https://codex.www.ads-software.com/Plugins/Mail