Always difficult, if not impossible, in my experience. Often it is hard to even purchase a full-fare business class ticket. On some flights that I am looking at for later this month, the flights are completely sold out (business & economy). I guess that doesn't leave much for the frequent flyer redemptions. I think that South America is one continent where one-world is very underepresented - I think there are only 5 one-world flights per week to South America (LAN SYD-AKL-SCL). I'd be interested in hearing anyones suggestions as well.
There will be more options available if you travel via LAX and then either DFW or MIA. The LA/QF flight via AKL to SCL is always hard to find premium cabin awards. Note that wen using QF FF points for a business class award, any 2-class AA domestic flights will be in economy and not in first class as you might first expect.