I don't know if that works, but I have gotten Captcha integrated into my Public Mail plugin. This is done by using the new Captcha 2.1 and not the old one (given up on that one).
As far as I can see it is in fact not possible to use the CB Captcha Plugin version 2.x outside of the CB plugin framework, ie in another extension.
The error that I get most of the time is this:
Fatal error: Call to a member function get() on a non-object in /home/myaccount/public_html/mydomain.com/components/com_comprofiler/plugin/user/plug_cbcaptcha/cb.captcha.php on line 136
I was going to stick with version 1 anyway - I found a version 2 download somewhere but I am not a doc subscriber and I'll play fair. However I think it would be nice if the CB team would produce a version of the plugin that does work with other Joomla extensions and comes with a working example. Otherwise the popular extensions are doomed to mess up websites with a variety of captchas.
