Known cases of miners withholding blocks when used as randomness? There are some applications that use the blockhash as a source of randomness, often with a payout similar to a lottery. I've heard of attacks where it would be more profitable for a miner to withhold blocks and lose the block reward, but increase the chances of them winning the lottery payout. I've heard this has actually happened, but never seen details.

Are there any known instances of such a withholding? Are there numbers on what the payout would need to be relative to percentage of hash-power the miner controls?
You're probably better off going to some ethereum venue, as ethereum has been rife with broken schemes like this-- riggable "fair" lotteries and such.