Rather than signing dates, they should sign a hash of the block header that was mined 10 minutes ago. This proves it was impossible for a message to be signed by creating plenty of msgs or whatever. It's exciting seeing an early Bitcoin signature, thanks for the entertainment OneSignature.
this can be "fooled" too
My comment was referring to a single singature, not a chain of signatures. I'll comment a bit on the chain of signatures though. You can always encode a chain as a linear chain of outputs which can be proven. For instance, people could "define" (I put it in quotes because it's a social construct) the coinbase address to own the block PoW.
This PoW could in theory be agreed on to be transfered with a chain of signatures defines as a chain of outputs. This should work as a concept. The PoW obviously isn't in the address at the head of the chain, but it was never in any of the historical addresses. It's there because of our agreement and the fact that nobody could've tampered with the transfers.