If golang is your thing, take a look at this[0] or that[1]. You can probably find similar ones with some quick research.

The neat part is that you can easily pull down someone's github keys and automatically authenticate them against that identity. You can follow up with an email/password registration as a backup identity provider, if you really want to push the envelope (and save the public key for future automatic authentication).

Of course, if you're just looking to hack something together quickly, you can simply force a specific command to be run.

[0]: https://github.com/shazow/ssh-chat [1]: https://github.com/gliderlabs/ssh