Invoke Mollom CAPTCHA directly
I'd suggest you simply integrate with mollom to expose your form.

You just need to implement two simple hooks and you can also configure if it should be CAPTCHA only or analyse some fields.

Hook documentation:


hook_form_mollom_list()
hook_form_mollom_info()


As an example implementation, you can have a look at my patch against Privatemsg: http://drupal.org/node/720038#comment-3501758 or have a look at these: http://drupalcontrib.org/api/search/7/_mollom_form_.