It doesn't involve cryptography, but mastodon has for at least a couple of years supported link-verification in profiles (it basically checks if a link back to your mastodon profile exists on a page linked on your profile), so a linking to a page that only you credibly control (say, a personal website) is the de-facto system of decentralized user-verification on mastodon.

Edit: supported since 2018 https://github.com/mastodon/mastodon/pull/8703

I think most people in this situation do the same thing people do on Twitter when it suspends an account for nonsense reasons like telling a Nazi to eat paste: make another, make a post, share it with a few friends off Twitter/Mastodon, have them RT/boost to circulate it within their circles. This seems to work pretty well.

Cryptographic certainty is fun to think about, but sometimes you just need people.