[dltmrcd]

It may not be worth the complexity to give users the choice. If I were to issue keys to guests this way I would want my revocations to be immediately effective no matter what. Guest keys requiring a working network is a fine trade-off.

[azernik]

You can have this without user intervention - have the lock download an expiration time with the list of allowed guest keys, or have the guest keys public-key signed with metadata like expiration time.

If the cloud is down, revocations aren't going to happen instantly anyway. (Although you might be able to hack up a local WiFi or Bluetooth fallback.)