Comment by jcgl
Not to split hairs, but that sounds more like plain hackery rather than proper extension. Let alone a viable future. I don't doubt what you say that it's possible in the most technical sense of possibility. But actually possible, in this world where we live? No, doesn't seem like it.
I might be overlooking something, but if Mastodon were to decide to add an input field to your settings page where you could enter your domain, along with instructions on what static file to upload where (ie in .well-known), I wouldn't be surprised if the rest of the ecosystem were to follow.