Interesting. Thanks for sharing this.
It is already possible for users to transfer fiat between one another, using the [wallets_move] shortcode and the accompanying do_move JSON API call. However locking is not possible at the moment.
What you are describing is an escrow service. This is not in my plans to implement at the moment.
Could what you are proposing be done simply via the Exchange extension? Functionally it is the same, except there is no restriction on who you are buying from or selling to. Are you trying to create an OTC market on top of the Exchange’s markets?
Finally, I already plan to add a way to store bank transaction IDs with fiat deposits (and later withdrawals). This will be out soon.
with regards