It would work if you encounter a movie that you wanted to watch, decide you want the money instead, and you report to Custos.
This is not how piracy works.
Instead, a pirate knows the value of the good they have. They know it's worth more to their reputation in the community than $5-10 dollars. By setting a bounty on it, you'll just have others willing to pay more to leak it themselves. This is an experiment, not a business.
Effectively it does the same thing but I think psychologically it's pretty different. For one thing you know that the bitcoin transaction will be very hard to trace (you can't be betrayed by your IP address or something like that) and you don't really rat anybody, you're just moving bitcoins. If you don't have any direct relationship with the original leaker (say, you downloaded the file through bittorent on some private tracker) there's really no incentive not to claim the bounty. It's free money.
I wonder what happens if a legitimate recipient of one of these copies decides to claim the bounty without actually leaking anything. Could they be in trouble? If people accuse them of leaking a copy and they show that they have control of the coins, will they be off the hook? Do they have to sign something that forbids them from using the key?
Of course it's one of these smartass "hacker technicalities" that may not hold much water in a court of law.
The other counter-attack would be to distribute the scanning software to all the people who got the screeners so that there are a bunch of transactions and no way to know which one leaked vs. just took the $5. Probably too small an amount to get many takers, but it would muddy the waters even if only a few did it.
What do you mean “instead”; draining the Bitcoin wallet identified in the bounty watermark doesn't make your copy (or any identical copy) stop working, it just let's Custos know (by reading the blockchain) that the original from which it was copied had the corresponding Bitcoin wallet drained.
There may be a trade-off about future content from the same root source, but not the current piece of content.