Speedbird1T and MartinS,
We are not talking about airlines that "position" their crews on other airlines. In those cases the airlines purchase tickets and the crews fly as passengers whether in uniform or not.
"Jump seating" is when a pilot from another airline shows up at the gate (often without prior notification) and asks for FREE SEAT.
This is still in practice with many airlines.
In the old days this seat was understood to be in the flight deck and they could sit in the passenger cabin if the airline allowed it. But now the flight deck is supposed to be secure and even pilots must ride in the back.