@sam Librem.one has an interesting approach where the same account can be used for mastodon, email, matrix. You'll need all services hosted in the same place. I don't know what they do in the backend, but is this something you were thinking of?
@dean I don't know what they do, but the fact that they all have to be hosted in the same place makes me think they're just provisioning different accounts with the same username on each. Not quite the same as a single account on one instance with all the posts that you can log into with various clients that create different types of posts.
XMPP based social networks do this a lot better (one central account I control, lots of different front-ends that all show data and connections differently), but they don't do authorization well (you have to type your username and password into every separately website and give them access to every other sites posts, messages, etc.)