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.