Add a callback 'pre_signup_requests' executed in the beginning of login/signup.php

1. Enable self-registration (Site administration>Plugins>Manage authentication)
2. Make sure a user can register without problems
3. Log out
4. In any plugin create a callback

   function PLUGINFULLNAME_pre_signup_requests() {

   throw new coding_exception('boooo');

   }

Try to signup again, you should get an error when hitting the login/signup.php page