Is it safe to store a sha256 hash of my BIP39 seed on the blockchain?
You could and it's probably not a direct security issue, but it's a senseless waste of time trying to find it again. Keep in mind that doing this destroys the PBKDF2 key stretching used in the seed needlessly. Electrum stores a checksum with their 12 word seed (making it a 13 word seed) if you really desire that functionality, though it's clear if you have it right when the money in that wallet is restored.