In order to waitlist you'll probably have to make (and pay for) a booking for another date, then ring and see if you can waitlist. I'm told that QF won't waitlist for Y, and that LA sometimes do.

I'd recommend you don't bother trying to waitlist, there's little chance of getting it and you should have firm(ish) plans before you leave.

Also if you waitlist and Y seats become available then it's unlikely that they'll put them back into the L bucket if they think they can sell them in a more expensive bucket.

There's plenty of things you can do/see in central and south america while waiting for your flight. For example LAN sometime have some really good deals on flights. Also the quality of buses is high in most places.

Last year we were hoping to change our SCL-AKL flight during a RTW, in the end it introduced too much uncertenty so we booked (from an internet cafe in easter island) flights from SCL-MEX-SCL and enjoyed a great 6 week break in central america which we hadn't originally planned on.