Very interesting. I am pretty sure that you can configure MWG to speak with a captcha backend. Googles Recaptcha (http://www.google.com/recaptcha) has a Web API that you can query, so it should be possible to use external lists to communicate with the backend for validation of the input.
Unfortunately I have not set it up and don't have any free time to play around with it. However if you want to give it a try I am interested to see the results :-)