Django: Customizing the comments framework: accessing context user in custom form?

Question

the goal is to render a comment form with an extra recaptcha field if the user is not authenticated.

I'd like to extend the comments framework, making the contrib CommentForm dynamic, so the constructor checks to see if the context user is authenticated.

But it doesn't seem like I can get the context user in the form without passing it to the constructor directly. Which would require rewriting new template tags, which I don't want to do.

Solutions?