Most airlines don't include climbing gear as 'sporting equipment'. They mean golf clubs, maybe skis etc. Sometimes you can convince them, often not. In the UK it's a huge problem where lots of climbers use (very) budget airlines to go ice climbing in Norway or France/Italy.
If you're flying from Australia to SthAm with LAN/Qantas you'll be on the two piece system anyway, ie. 2 x 23kg. In theory, if you stay on a LAN flight internally, on the same ticket, with no stopover, the 2 x system applies domestically as well. Sometimes the person at the counter may not know this, or may choose not to remember, so you may have to remind them. If you break your journey though, the domestic allowance technically applies.
AerolineasArgentinas on the other hand I think only have a 20kg limit Sydney-BA. For regular expeditioners excess baggage is just a standard part of the budget. I have flown Sydney-Santiago-Punta Arenas with LAN/Qantas ten times in the last decade, usually with about 100kg or more of expedition baggage - once that meant seven big duffels, plus carry-on. I never used to bother asking for a discount or anything, just factored in having to pay $750 or so (5 extra pieces at $150 each) at the counter. Fun times
I've never had any problem losing luggage into or out of South America. I would have thought Buenos Aires would be more of a problem than Santiago. The latter is relatively modern and efficient now.