Requiring a secret word to register with Self Registration Module

[patty]

Hello

We are using the front end users module and the self registration module. We need to only allow certain people to register. Our solution was to give them a secret word in a publication then have them enter it in one of the fields on the registration form.

We tried editing the captcha stuff, but that is really way more than we need. We would want it to be something simple like the please prove you are human thing here:
http://www.theysuck.org/bbpress/register.php

But say enter the secret word and only allow them to register if they provide our one secret word. Any ideas?

Thanks in advace