I suspect that there's a lot of eating crow and bargaining with rivals for space on their flights to help clear the backlog, but each incident is going to have unique elements to resolve in it...
If it's only one airline down, you can get away by buying tickets on competitors.
If it's multiple airlines, or the upstream reservation system, or a local meteorological incident, or an airport wide issue, etc... there are no flying plane.
It's standard practice for airlines to fly each others' crews around even during normal operations (often at no cost).
For example, the overbooking that led to the beating on the United flight was the result of an aircrew from another airline being booked on at the last second.
I would imagine that when the shit hits the fan like this, the other airlines are very sympathetic and will do whatever it takes to get BA personnel where they need to go. After all, next month it could be their own systems that are down.
(assuming it's not one of the shared services causing a global outage - not much you can do when everyone is down)
> For example, the overbooking that led to the beating on the United flight was the result of an aircrew from another airline being booked on at the last second.
That's not quite what happened. The flight was operated by Republic Airlines, on behalf of United. Republic bumped the passenger to make way for more of their own crew - it's just that crew was going to to fly a Republic flight operating under another, different, carrier. But both crews were employed by Republic.
