LAX-EZE in F:

Did this on a OW about 3 weeks ag.
Travel agent issued the ticket LAX-EZE (one coupon) in F (or whatever the correct letter for First was).
Reservation was LAX-SCL in F and SCL-EZE in C.
Got both boarding passes in LAX, Coupon was collected and the second boarding pass was stamped. Got Row 1 for both boarding passes (you have to ask for it - LAN uses the same config - not the same plane - for SCL-EZE, the F row is still there - but C service).