This may be way off base, but could you instead of having a plugin just have the keybase user insert some string in their profile about_me? (https://meta.discourse.org/my/preferences/profile) Then no plugin would be required and people could have a proof on any Discourse site, not only those who had heard of KeyBase and were willing to install a non-standard plugin.
Edit: Oh, see Discourse Keybase Proof where this discussion seems to be happening.