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
This is how eg anoncvs works. After that point, it's just a matter of writing the application you want to run, with stdio being connected to your user.