If you log in with the "public terminal option" and don't bother to fill in the captcha, you should be able to submit a comment. For short comments, this works just fine....
However, if you take too long to compose a comment, the login will time out. Then, when you press submit or preview, it will show you the preview screen with username/password blanks, but instead of submit/preview buttons, it will say "Sorry, you failed to prove your humanity. You must start over."
Once you have successfully logged in once, it should set a form field in subsequent generated preview pages that says that if there's no login cookie, you get prompted for username/password and are not thrown the "failed to prove your humanity" line.