Not an expert, and this is limited in what you could use it for, but for gambling purposes, couldn't this be dealt with by using the next X hashes after the bet

Not an expert, and this is limited in what you could use it for, but for gambling purposes, couldn't this be dealt with by using the next X hashes after the bet is placed, where X is a large enough number that the likelihood of the better being able to produce that many successful blocks in advance is nil. So, at the simplest, you take a bet, and then use the next two hashes for your random seed to get a result. Even if a miner found a solution for the next hash and withheld it long enough to make a bet, he wouldn't have enough information to do better than random chance. And as I understand it, the entire blockchain relies on the fact that one miner shouldn't be able to generate two blocks faster than the rest of the mining pool. Increasing from two grants even greater certainty. Of course, the trade off here is that you have to wait for X blocks to be generated after a random number is requested and before being able to use it. So, your gambler has to sit around a bit to find out if they win, rather than instant feedback. (Of course, for gambling, you can hide this with some pretty animations of a roulette wheel, dice, horses running, etc.)