My experience with long haul GUCs is actually that I’ve cleared every time, but not until somewhere between 30 and 3 days before the flight. I book unpopular midweek flights - tues/wed primarily - where I can claim a great Comfort+ deal at booking, and then I hunker down and wait. This strategy hasn’t failed me yet.
If you require confirmation of J at booking to SCL and already know travel dates so far in advance, I recommend looking at flying AeroMexico. Despite reports to the contrary here on FT, I’ve had nothing but good experiences with them. They aren’t CX and you shouldn’t expect premium catering, but the food is comfortable and tasty, and their 787 seats are totally fine. The usually very reasonable price for paid J more than accounts for the ordinary catering.