I've had no problem at all booking LAN flights on britishairways.com with Avios, and have done the JFK - SCL route several times. I've had to call BAEC for intra-South America routes (BA's booking tool doesn't recognize many/most cities that LAN, LAN Argentina, etc. fly to in S. America) but they have always waived the telephone booking fee if I pointed out why it couldn't be booked online.

One obvious caveat: booking so far in advance ensures that either you or LAN will have a schedule change. If you change your mind, cancellations on BA's site are easy (and they don't cost more than the tax you paid upfront), but changing dates is not very easy. When LAN changes its flight times or cancels a service, they have a longstanding nasty habit of not contacting passengers directly (presumably relying on BAEC to pass on the news). In other words, during the 330 days between booking and flying you need to periodically check that LAN is still planning to operate their flight. Forewarned is forearmed.