People have been trying to watermark digital media for decades, when there was (still is) a very strong financial incentive to get it working. It never worked. I don’t think it ever will work.
You are confusing a digital signature for evidence of anything other than an attestation.
If you create a digital record, then sign it, then that signature is only an attestation of may claim you make, not evidence of that claim. That is the problem with relying on technology to establish trust - the moment you attach an economic benefit to a technology you incentivize people to circumvent it, or to leverage it to commit fraud.
You can still declare success if you lower the bar to "we can catch leaks/pirates and in particular we can know which internal folks should no longer be trusted. ... as long as they don't attempt to circumvent the fingerprint"