I know there are a bunch of #retro computer Mastodon apps, but one I've not yet seen is a #BBS door interface.
I figure you could go two ways, have a local mastodon instance and it sort of works like the mail systems (though I doubt they'd scale well). Or alternatively, you get a token to login to an external instance (similar to elk.zone) and it's just a text-based front end (like wrapping https://github.com/ihabunek/toot). Perhaps one already exists?