The least significant bits of a bitcoin PoW hash behave pretty randomly.
You could either bet on a fixed future block height, or on the first block past a given future timestamp...
That's actually very easy to control. Just pay a high transaction fee. The nonce comes from a PRNG that doesn't have to pass many randomness checks. Your proposal really is no more random than a counter based SHA256 PRNG except with an awfully high sample latency.
The least significant bits of a bitcoin PoW hash behave pretty randomly. You could either bet on a fixed future block height, or on the first block past a given future timestamp...