Also, as I looked a little bit into Keybase I learned that they don't support any protocols that don't have public profile-like pages. So a pure messenger wouldn't be supported by Keybase.
See the blog post and the spec that details the changes to implement: https://keybase.io/blog/keybase-proofs-for-mastodon-and-ever...
I presume a pure IM system would have to implement some web gateway at the server level.