[Fnoord]

True, which is what a notary is for. You could encrypt the data to be leaked at a notary, with the private key split using shamir's shared secret among your beloved ones (usually relatives). If all agree, they can review and decide to release the whistleblower's data.

[Kon-Peki]

This statement confused me, but according to Wikipedia the job description of a notary is different in different parts of the world. If you live in a “common law” system (IE at one point it was part of the British Empire), it is unlikely that a notary would do anything like what you are saying.