Frankly, my part, I would find it less inconvenient to digitally sign a challenge with a Bitcoin key or PGP key, and paste the result into a textarea. The CAPTCHA is that much of a deterrent for anyone who is accustomed to crypto, and has limited time.
Any which way, if
any popular forum has users who can handle public-key crypto, it should be Bitcointalk.org!
Someone could make a tor/firefox plugin that could handle signing challenges for users unaccustomed to it. If an account started DDoSing the server could ask it to sign more challenges with a time delay before them.