It would have to be from a miner or pool with a large enough share of the hash power to actually be confident in finding blocks in a given range. Unless a coordinator buys futures from many different miners to give the same effect.
Discussion
It would definitely drive centralization but as you say, it would not have to be the miner or even the pool that offers such a service. The service would have to get access to some blocks per day at least to make sense but that's like 2% of the hash rate. You don't need some 50% for that.