Can you add more details to help narrow the exact situation you are asking about? If there is an expat who works for Cathay or Emirates for example, and is travelling domestically back here in the states then there is no possibility of using a JS pass to ride.

Those two carriers are not in CASS, so riding in the actual flight deck jumpseat on US Airways would not be allowed.

Additionally, those two carriers are also not on the US Airways reciprocal jumpseat list, so an expat employee (pilot) of either of those carriers being able to ride in the back using a JS pass is also not possible. This is similar to how any pilot from a foreign air carrier would be handled, when their airline is not on the reciprocal jumpseat list. Doesn't seem to matter about the birthplace/citizenship/expat status of the employee, it's more focused on the established agreements between the various employing air carriers to allow or disallow various JS travel options.