Your best chance may be Iberia. A friend of mine, also an intrepid airman from Miami jumped from KMIA to LEMD. He had to wait in the terminal to talk to the captain before talking to the personnel at the checkin counter. Same applied from LEMD back to KMIA. This was about 6 or 7 months ago. Other than that... there are no jump seat agreements in the sense that we have in the US.