Vacation flag and clan challenge settings

I just learned that the vacation flag automatically sets your clan challenge setting to unavailable. That may be useful in many, maybe most cases, but I think it should at least be possible to override it. We just had a 20/20 challenge start several days later than planned because one player was on vacation. It's a 7-day timeout, so it wouldn't have been a problem for that player if the challenge had started while he was away. I would guess that such a situation isn't that uncommon, especially with big challenges where it can take some time to get all players available at the same time. Another idea would be to allow players to make themselves available for a specific challenge in advance, so that the challenge can be started even if they have made themselves unavailable for challenges at that time.