The law, even the constitution, has limits. The remedy lies in the hearts of men. Choose your surrogates with care, and choose surrogates! My advice is to put NOTHING in writing, because the written words can be misconstrued. Trust you surrogate to assert ethical judgement in the face of an unethical legal and medical community. Not all lawyers and doctors are unethical, but it's the "safe bet" to assume they are.
And if the surrogate become incapacitated with or before or shortly after you? Or you have no close family left you can trust? Or the surrogate is sweet now but turns sour later?
Putting nothing in writing led to Terri's murder-as-we-watch.
Surrogates have their place, but I would want to see a national database for advanced directives. Index them by name and SSN and fingerprints. And the directives must be in plain English, and a Federal law governing what they mean, and the requirement that all be sworn on oath.