If you like you can PM me (or post here, as you wish) your intended dates and I will look at the spanish version of the LAN site, to see what they say about it.

Edited to add: no need to PM me, I have seen it on lan.com and can confirm the seat reservation restriction merely means your seat cannot be selected by you before the day of departure, as it is assigned at the airport; but rest assured your seat is confirmed. It is NOT a stand-by fare.