I flew R/T SCL-IPC last March. The planes are definitely fuller, so chances of Y sitting in J section are slim to none. The plane was a 3 class flown as 2 class but row 1 was full both ways and J section was ~80% full.

This flight continues on and returns from Tahiti with a stop on Easter Island, so it's not just people flying to/from Easter Island.

LAN allows F and J seat assignments at any time, but I believe Y is not assigned until day or 2 before the flight at the earliest.

Also, since it's really an international flight, many passengers get confused and go to the left/international departures, even though they are not leaving Chile if their final destination is IPC. After going through security they find out the passport control agent will not allow them through and they have to track all the way to the other side of the airport and do the security check again to enter the domestic terminal. Then it's a real schocker to them when they find out they can't get to the gate because it's the first gate on the international side, behind a glass wall which has lock doors. You'll see the gate, the plane and some people on the other side but don't panic, and just find a seat where you are. When boarding commences, the security people will open the door and you will be allowed to board.

The ONLY exception is if LAN invites you to their departure lounge, either because you are flying in J or you have Sapphire or Emerald OW status. With the invitation, the passport control agent will allow you through on the international side and you will actually be on the "right" side of the glass wall before boarding.

I hope this makes sense. Basically non-status Y passengers go to the domestic terminal and have to wait there until boarding starts and OW status holders or J passengers or LAN Lounge invitees can go to the international terminal and enjoy the lounge(s). BTW if you do have lounge access, I actually prefer the Admirals Club which is just couple of minutes walk from the LAN Lounge. You can actually check out both.

Have a great trip! I enjoyed myself on Easter Island. Did a lot of exploring on my own. Met some great people. Actually I partied with the Chilean Assistant Minister of Culture on my last night there as he was staying at the same hotel (motel) and the staff and all the guests were invited to celebrate his birthday. Most of the guests, myself and the minister flew back to Santiago next morning with huge headaches......