Well, I'm not sure I'll be able to do it now, I'm quite new to Joomla...
Naturaly, I would redirect the link in CB Login menu ("Get Account" or so) to my extra script wich I believe I can inherit to Joomla (add "defined '_VALID_MOS' or die" stuff) and if the test is succesfull, send it back to CB registration page. But if I want to be serios about security, I will have to modify this page to check for control code (generated, defined by my, passed to CB join-form).
That's mean I will have to modify some CB files. So I have maybe stupid question - when creating plug-in, I will have to replace (during installation) those CB files, which should be modified?
I think my skills are not good enogh to create Joomla backend dialog to allow administrators to change the test questions & required success level, but I can put it into db via phpmyadmin and figure it out later (and so eventual users can :-D).
OK, I'm gonna have a look on that right now. I will start with porting the test scripts from my old [own] CMS.