I'm not sure if it is so, but I would hope browsers attach an expiry date to each hash, not each hash set, and that hashes can't be revoked simply by later omis

I'm not sure if it is so, but I would hope browsers attach an expiry date to each hash, not each hash set, and that hashes can't be revoked simply by later omission. i.e. if you have keys A and B, and an attacker takes your domain and sets a header solely for the hashes of keys B and C, I would hope that you can still use a cert built using key A until the expiry.