I agree with Debra, and I think that having a physician sign a document is not the right path to take. Even if you can find a legally binding document, I would be surprised if your physician would agree to sign it, and just the act of asking them to do so would likely make any working relationship with them very difficult.
But in reading your original post, I don't believe that you, as a coder, would be liable for medical necessity issues - medical necessity is outside the scope of coder training, so I'm not sure what you mean in saying that you have 'proven medical necessity does not exist'. Only a physician or peer with clinical training can make a definitive determination about the medical necessity of a service provided to a patient, and performing medically unnecessary services is a medical practice issue, not a coding or billing issue. If a payer were ever to challenge medical necessity, it would be the physicians who would need to justify their decisions to the payer's medical directors, and would usually not involve a coder. As long as the claims you are billing are supported by documentation and you are not knowingly submitting claims for services not rendered or not supported by the medical records, I can't imagine that you would be held liable for this.