I had it happen to an old Campy Record hub on my old Trek, touring in France, when I was 14. Basically identical to the OP's failure. We were in the town of Roquefort (where they make the blue cheese) - on Friday afternoon, of course, with little prospect of a bike shop being open on the weekend. This was 2002 or so.

Had to rig a bunch of spokes twisted and tied together in order to make the wheel true enough to ride the next two days until bike shops were open and we could buy a new wheel. I remember feeling really bad for slowing down the family tour, and my dad lecturing me about "riding light" and how hard teens are on components, or some such nonsense.

Really, the family shouldn't have equipped me with Campy racing gear for loaded camping. Looking back, I'm glad the flange cracked. Had it held and had I continued on that hub, I'm confident the axle would have broken - and aren't those Campy axles a weird thread pitch or something? I can't imagine trying to find such a thing in France in the early 2000s, knowing none of the language at that time. I remember the Monday after, my mom managed to pantomime a SRAM freehub and 9-speed cassette at the bike shop guy, and then my dad spreading the frame as it sat upside down on a picnic table near our campsite that night.