In Proof-of-Stake, can a block creator fudge the block hash? In Proof-of-Work, a block hash is costly to manipulate (say, wanting the hash to be even, or have the hash of the hash smaller than some number) since there is a lot of computing power put into selecting the right header to produce a valid hash.

I'm wondering - are the block hashes in the various Proof-of-Stake implementations easier to manipulate since the hashes aren't bound by PoW mining? Could a PoS miner spend some computational time to generate many valid blocks and pick one to use, or are there some other constrains preventing block hash fudging?
Absolutely. ECDSA signatures at not unique, they can always be permuted by the signer at will by re-signing with a new nonce. The creator of the block can also choose to reorder or omit transactions to alter the block and then re-sign this to get a new block hash.