Putting CAPTCHAs on their own page?

We need to put a captcha image on our ASP.NET MVC 2 based website. We chose reCaptcha and bu开发者_JS百科ilt it in using the way described by Derik Whittaker. The idea there is baiscally to build some abstractions and all you need to do is decorate your Controller with a [ValidateCaptcha] attribute. This works all fine.

However, we have a lot of form-widgets in different pages and I don't want to have the captcha floating around everywhere. So I'd like to implement it the way StackOverflow does: Submit a Form -> Challenge Captcha -> Submit Captcha -> Perform Action on original form data.

Now, how do I redirect the user to the captcha page while keeping the originally submitted information?

I thought of some very ugly hacks (hidden fields w/ base64 encoded form data, etc.) but I think I'm missing something obvious. On the other hand, this sounds as if I wanted to do something in a stateful manner, and I shouldn't?

Session or persisting model state on the client comes to mind.