Now by the end of 2017, I can suggest this:
https://www.percona.com/doc/percona-server/LATEST/management/pam_plugin.html
Percona PAM Authentication Plugin is a free and Open Source implementation of the MySQL‘s authentication plugin. This plugin acts as a mediator between the MySQL server, the MySQL client, and the PAM stack. The server plugin requests authentication from the PAM stack, forwards any requests and messages from the PAM stack over the wire to the client (in cleartext) and reads back any replies for the PAM stack.
It is NOT tested, and I have never worked with it, I wanted to suggest it as it may be good.