Dir auths should compute Guard WFU using the consensus, not private history
Currently directory authorities track the presence of each relay and keep notes about their view locally. Then when it comes time to vote about Guard, they look at their notes and decide what fraction of the past interval the relay was up for.
But it doesn't matter anymore to clients whether the directory authority could reach the relay for that time. The question as of the v3 directory design is whether the relay was in the consensus.
So it seems like the directory authorities should be basing their measurements off "is it in the consensus this hour".