After a night of thinking, I've not gotten far, but here is an idea:
In this computing space, all file types record how they were created, containing a history of edits and authors. (Authors could just be private/public key pairs, no sensitive information)
So a text file would record how each character entered the document, and who wrote what.
A bitmap would record what pixels were touched, in what order and by whom.
(This kind of CRDT-like data structure could have benefits when collaborating as well?)
Any procedurally generated content would be recognized as such, and a generated document's history would look very different to a document that was crafted over several months. One could even show a "generated" tag on files deemed to be generated.
If this was it, it would obviously be easy to fool the system by generating a plausable history. But maybe there are ways to protect against this? "Oracles" with unique random number generators that keep track of time, and edits being signed by those, somehow? If each "private garden zone" had its own oracle, and you had friends, your friends could sign the work you're doing over time, ensuring that the history is correct?
I'm probably reinventing a lot of bad cryptography concepts here, hehe. I know very little of security/privacy.