I think it’s better to return to the original topic. To summarize, the idea is to use some kind of signed object with expiration date to represent approval. This object can be inlined to make verification possible without making request to authority server.
To make approval revocable, authority can make it short-lived (for example, approval may expire in 30 minutes). Expired approval can’t be verified offline, so authority can revoke it by returning 404 in the same way it can revoke approval in the form of Accept activity.
The main advantage, in my opinion, is the possibility of offline verification.