If such a thing existed, it would have helped this guy: http://huff.to/kKIGDa

The important part is in the grey box. The guy went to a notary public and legalized a statement that he could later use in court.

You could take your idea further and actually charge users to have their submission automatically notarized, after they prove their real life identity, of course (there's already a startup that offers to certify users' identities, anyone remember the name?).