WHCA* turned out to be a bit too suboptimal for our use-case, people generally expected "perfectly optimal" routes to be used for aircraft, and they weren't even overly happy with most-optimal "for-all" paths either. We eventually implemented a relatively simple "AStar-3D", essentially just A* against a space-time graph, and it's greedy/FIFO -- meaning it's optimal for each aircraft at the time the aircraft runs it's path. That made people happy -- aircraft no longer did seemingly stupid things like "oscillate", or get "temp. stuck" for overly long periods, etc.
I had no idea cooperative path-planning was so damn difficult -- I remember estimating it as a 1-week mini-project initially. Wow, such naivety, and that's when you even have perfect information! Such a cool domain, tons of respect for the work that's being done here, even if there are some tricky/ethical aspects that are going to come into play eventually, inevitably. :)