>key distribution
GPG key servers?
All the moving parts probably exist, but so far no one has pulled it together into something that a random person off the street can use.
Its not the problem JMAP is trying to solve.
The keyserver protocol includes commands to include keys in the keyserver network. How you send keys depends on the UI of the tool you use, geeks will do this:
gpg --keyserver <keyserver> --send-keys <fingerprint>
https://mail.gnome.org/archives/gnome-announce-list/2014-Nov...
The average user doesn't use GNOME or Linux though :(