I can assure you that staff can most definitely fly J internationally.
You're given an economy BP upon check-in. If there's availability and provided you are in compliance with the staff travel dress code that BP will be swapped at the gate for one in the J cabin.
Ewing,
Thanks, but I never said staff can never fly in J internationally. You are correct regarding the process, however, prior to receiving your J BP you need to have booked in J and paid for it through staff travel. You CANNOT just pay for an economy seat and then rock up to the counter and ask for a J seat.
Happy Landings.