And now for something completely different !
Set up a passcode system where viewing the watermarked file requires entry of a unique code known only to the preview holder. Log the viewer's passcode entry event on a remote, well secured database (not Sony's).
If there is no internet connection available, send an OTP to the viewer's mobile phone. If there is no cell phone coverage or mobile phone, tell holders they must wait for snail mail or call a special number. My guess is they will find a cell tower PDQ.
This could provide a simple, easily enforced. tough-to-beat audit trail. However, it would also require a foolproof self-destruct mode. If the code is incorrectly entered three times (or more, depending on how intoxicated the preview owners tend to be). Also enclose a stenographic method that ensure any tampering or removal immediately destroys the file's contents. That last feature requires more analysis but there should be a way.
Of course, use of the correct codes followed by server validation would still allow at least one screen scrape (which is the fundamental flaw of any audio/video copy protection). But that still becomes prima facie evidence of insider knowledge and diffusion. Particularly if you want to be a real b*rd and force a passcode change after each viewing and then communicate it to the OTP device holder. Providing more bread crumbs to follow...peck peck.
Now, to really close the circle, make sure the preview DVD is required to phone home a set number of times before automatic self destruct. Or send the self destruct code after official film release. LIkewise, you could optionally release the DVD from self-destruct mode if the holder continues to use the generated OTPs and/or always phones home with the DVD.
When the DVD disappears from the auditing network and shows up on bit-torrent, you can audit the DB and find out which DVDs have either
a) not been officially returned to the MAFIAAA mother ship
b) not phoned home since the film's appearance on bit torrent
c) not been self-destructed
d) not been released from self destruct mode
That number should be fairly small, unless pre-view leaking is truly rampant.
It would narrow down the list of suspects in any case, and might discourage leaking, assuming that is what Hollywood really wants.
But, to be honest, I don't even know why I would want to help the film industry.
I'm still waiting for my shipment of champagne, hookers and hot-tubs. And every time I reach out, they say "don't call us, we'll call you".
Coked up gits, all of them. That's why the kind of security systems used in banking, mil-sec. many web pages, etc. will never fly there. You have to be reasonably coherent to use them.