Wait, how are they doing that? My mental idea of NAT is that it dyanmically links (address × port) pairs to local addresses. What would be the "port" in the case of flight numbers?
origin,destination tuples.
UTC Date + Time of flightplan takeoff
If that would be usable and could be assigned at sufficient precision the flight number would not be necessary at all!
NAT for airline flight numbers would fix this problem and improve security to boot!