You have to clarify when using the jumpseat term as it applies to international travel.

Can an offline pilot JS on (insert offline carrier here) to get a ride to (insert international destination here)? Of course they can. In the above case a CO pilot get on a UAL flight from say SYD and go to LAX using the JS privilege as long as there is an open seat in the back.

Are they allowed to occupy the ACTUAL cockpit JS when going to (insert international destination here)? Thats a WHOLE different deal when referring to U.S. legacy/major airlines. ACMI's handle it differently.