It would be extremely useful to me if an AuthSource subclass could supply a CAPTCHA challenge image to the end user (presumably from the authenticating back-end), and subsequently get a CAPTCHA answer back.

I've been working on a "ClientLogin auth source":http://github.com/EvanK/redmine-authsource-clientlogin which has the drawback that sometimes Google decides to issue a CAPTCHA challenge. When this happens, there is currently no way for me to pass this on to the user, so authentication simply fails until the challenge expires.