Borrowing elements from another fic doesn't stop it from being a fic of the original work. I think you should remove the goodfic from the equation.

So, it's a badfic of the canon rather than a badfic of a goodfic of a canon.

Well, I think for something like that, it could really be quite subjective and differ from fic to fic. If the goodfic doesn't differ too far from original canon, then you could easily use the real book as the official canon. If the goodfic is sufficiently different from the original (such as having OCs, following the life of a minor character, or being AU itself), then it might be better to use a combination of the goodfic and the original work as canon. It seems like it would be up to the person sporking the fic. I know that some people would say to only use the original work as canon, but I think if a story has earned the title of "goodfic," then the work that was put into it deserves to be protected, especially if another fan decides to muck about with it and completely destroy it. Again, though, it should really be up to the person's discretion.

Has anyone ever sporked a fic like that anyway?

The PPC seems to define a canon as an original published work. I think that a badfic of a goodfic would still be tied to the original canon, although I suppose it would be an alternate version of that canon, like a bookverse versus a movieverse.

However, that's a rather controversial point.

For example, what if someone writes a badfic about another badfic? Are we supposed to spork it as-it-is, or should we pass on the charges already present in the "canon"?