EXTCODEHASH to prove commit block

[deluca-mike]

Just heard of Submarine Sends today, and read the documentation and smart contracts, so forgive me if my question or suggestion is naive.

Instead of using proofs or challenges as a mechanism to retrieve or trust the commit block, why not have the contract itself store the block number of the latest/last commit by the user. This can either be done via the constructor when it was deployed, or via a payable commit function.

Now, we want to be able to trust that the value of the public commitBlock was written to appropriately, so we just use EXTCODEHASH to check that the contract code actually does exactly that. We know what the contract code should always look like, given some fixed compiler version, so the deployed code's hash should be enough.

Am I missing something?