I don't know if I have seen ReCAPTCHA specifically used, but I am aware of people who have implemented CAPTCHAS for the login portal. If you can implement it in such a way that you can offload the checking to another server, I think that should work.
If anyone has a sample custom page with Captcha or ReCaptcha working, a post of such customization would be greatly appreciated.
ReCaptcha does offload the validation to another server. The problem that I have is that there is a Java Serverlet/JSP that needs to run on the Juniper SA6500 in order to send off the response to validate the captcha phrase. I have gotten the Captcha to display on the custom login page, but I have yet to figure out how to validate the captcha entry (JSP/Serverlet) on the Juniper. (I am not a java programmer that knows where to put the code so that it will work.)
Here is the overview from their website. (Picture is Attached)