I just applied the external stored procedure patch to a branch of MariaDB and uploaded it to LaunchPad.
You can see the branch at https://code.launchpad.net/~atcurtis/maria/5.1-wl820
Note that this is not in any reasonable condition to merge into MariaDB. Hopefully we can engage in dialog as to how we can bring this feature properly to MariaDB, MySQL and Drizzle, hopefully making the plugins reasonably easy to port between them, but to be "native and maintainable" without getting in the way of future development. For example, some of the infrastructure I have used to implement table functions was removed from the MySQL 6.0 branch. It would be nice if contributers can see and participate in roadmap discussions; historically MySQL may be Open Source but it would be good to have Open Development too.