I've been thinking about something similar for a while now. Although my scheme involves cryptographic attestation and typing stuff with hands.

Quite easy to rig up a machine to type on a keyboard. I guess you could require a video of it being typed.

Then, the typist could simply be typing in an AI generated piece of text.

The only solution is to trust the person who handed you the work to accurately tell you the author, and then trust the author to be telling you any attribution.

I personally have earned this trust as people know anything I AI generate will have an Assisted-by: tag on it.