You could also require a signed letter that has been properly notarized. Again, adds an independent 3rd party with a verified identity and something to loose. And falsifying is a crime.
Notaries aren't accountable for the content of the documents they notarize. They're only responsible for making sure the signatures they are witnessing come from the correct people. They'd be happy to notarize DMCA takedowns, since it's nothing more than income for them.