Still I believe MIME would be the right place to say something about the Media, rather than the Transport protocol.
On a lighter note: we should consider second order consequences. The EU commission will demand its own EU-AI-Disclosure header be send to EU citizens, and will require consent from the user before showing him AI generated stuff. UK will require age validation before showing AI stuff to protect the children's brains. France will use the header to compute a new tax on AI generated content, due by all online platform who want to show AI generated content to french citizens.
That's a Pandora box I wouldn't even talk about, much less open...