If I said who I was waiting for something to come out about, I'd be crucified
Publish a hash so you can prove that you always hated them when it comes out, without anyone knowing in the meantime.
Here is roughly how I (an amateur) would do it:
write "I have always hated _____!" (not to short to avoid accusations of back solving by brute force) and some random padding (to avoid attackers brute forcing it), put it through a cryptographically secure hash, only publish the hash.
To reveal and prove it, you just publish the original message including the padding.
But it need some story, some kind of conclusions, be it humor, catharsis, a lesson or something else.
It is built up like funny internet comic, but leaves the reader disappointed.