Isn't that only an issue for a chain that's starting from PoS from the beginning? Ethereum already has a PoW chain that they plan to turn into PoS. If the ethereum protocol agreement is to switch to PoS at block N, then I don't see how you can create a valid-looking N+1 block without yourself putting a large amount of real eth at stake.
The reason this can’t work is because it has no cost to create a new chain from scratch. I can create a thousand valid PoS chains in virtually no time, and there’s no decentralized way for you to choose the one everyone else chooses.
The only way to solve this is to give special rights to some set of stakers.