Creating one for this industry would be almost impossible and largely pointless
I don't know that it would be all that difficult -- we could say we are an association and we would be. Custom writing Web sites could include the association's logo and state they use only NAFAW members.
At any rate, besides verifying native speaker/writer status, members could subscribe to a code of ethics that would include:
1. The standard disclaimer that the work product is to be used as a guide only etc. etc.
2. Members will decline rewrite requests when they believe that clients have submitted work product word-for-word as their own.
3. whatever else the members decide...
This is what other professionals do -- we could too (don't forget the coffee mugs!).