Fare is the key - LAN offers four different economy fares: Base, Base Plus, Flexible, and Full Flexible; Base fares do not allow for advance seat selection. LAN allows its Comodoro (emerald) elites to reserve seats on any fare, but does not extend that benefit to elite members of other oneworld airlines.

LAN also "reserves" many preferred seats for elites and these open up at the airport. Be sure to check-in at T-48, but be sure to ask about available seats at the airport. IIRC, LAN also does not allow for the exit rows the be reserved, so if you get to the airport early enough you might be able to snag one of those.

Have a great trip!