Yeah, I'm in South America now, and 700C stuff is pretty much unheard of, except for skinny racing bike sizes. I haven't hit Buenos Aires or Santiago yet, and I hate riding in big cities so I'm hoping to avoid both, but I'll be looking into the possibility of buying 700C touring tires if I end up in either.
Besides carrying everything down with me (I carried six spare tires starting in Mexico), shipping stuff from home is my only solution to this. It's really expensive, and a hassle to find somebody's home address to use (I really don't trust the "poste restante" hold-for-pickup-at-the-post-office system for big packages with important expensive items inside). Fast couriers like DHL will tell you they can get a box down here in 3 to 4 days, but two to three weeks is more realistic (I've tried it twice). Some countries are better than others in terms of customs and duty charges, eg. Ecuador is utterly horrendous, Argentina is pretty bad, Bolivia not too bad, and apparently Chile is great. No experiences yet with other countries.
So, I'd recommend getting a new bike built with 26" wheels and riding that down here, although you'd still have trouble finding high-quality tires here (Schwalbes, pretty much forget it). You can ride here with 700C, you just need extra money for shipping stuff and extra time to wait for it. Good luck and check out my journal if you haven't already.