I think it might have been a Defcon or Black Hat talk on ways of communicating anonymously. They certainly didn't use a web forum, it was more along the lines of posting encrypted messages to Usenet that were widely distributed and could only be decrypted by the recipient.
This was taken as evidence that perfect operational security is possible even against nation-state actors committing a large (but not indefinite) amount of resources.
Maybe there should be some sort of informal game, perhaps played with the major law enforcement agencies, with the goal being to avoid identification. It could be justified by asserting that it would aid in training investigation techniques.
It needn't hand over monetary prizes, just admission of success. Maybe it could trade pictures of cats?