[Pierre-Marie Baty]

You would need to adapt the code of the navmesh to consider not "walkable faces" but "flyable zones". In that case you can't use right away the polygons from the BSP file, you need to "space-fill" the map space with parallelepipedic volumes yourself. Really difficult, but perhaps worthy of consideration. It's not impossible. The Quake III Arena bot uses a similar thing: the "Area Awareness System". Look for it on the net, there is a PDF file somewhere.