If UA is paying these FAs anyway, would the assign a couple extra ones to work the remaining HKG flights and just give better service to customers and a bit less work to the normally assigned FAs? I suspect the answer is no, but it could make sense on a full flight of this length. Or given the risk of airport disruptions at HKG, it might make sense to have some FAs in reserve in case there are delays and other FAs time out (although in this case, pilots are likely to time out too, so having some reserve FAs to work might not matter).