I don’t think this is an unreasonable request - particularly given there are no plans to print sort codes or account numbers on the cards. It could maybe be achieved by having a button in the app that displays account details specifically for this kind of situation, rather than one to hide the balance.
I’m not on the CA at the moment, so I’m not totally sure if it still has this layout, but in lieu of a ‘balance hiding’ feature would it not be best to show someone the account details on screen from the ‘you’ section? 