My point is that BankID should have something similar for any BankID action.
They could still add such a feature of course, but they would need to inform and have the co-operation of services when someone else is using the ID, so it wouldn't be widely supported.
Person A initiates and signs request to delegate for Person B. Person B receives the delegation request and signs it, which produces a positive response. The response (containing Person B's signature= is then 'wrapped' in Person A's request and is only sent on the to destination.