Well no I didn't know that. I actually think you might have been misinformed by the LAN agent. Await someone to back me up on this but you cannot even check in online for LAN flights from or to the USA. I suspect nobody can select seats.

I have flown BOG MIA BOG dozens of times and I am fairly certain that you can only pre check in, and there is no seat selection for anyone. Last time I flew MIA-BOG I looked at the seat map a couple of days before and the plane looked empty. People got bumped off that flight, it looked empty because no seats had been assigned.

My guess is you can select seats at T-48 and wont have a problem