I haven't yet been in South America, but my overall impressions from many places elsewhere in developing world:

1) Surprising what you can get in the largest cities, e.g. Santiago, Buenos Aires, Bangkok, Bangalore, Delhi, etc. Becoming more of a global market.
2) While tires or wheels might be found in the countryside, they aren't necessarily the highest quality or durability. If one is traveling with loaded touring might be possible to limp to next place, but might well be replacing again in larger cities.
3) Failures are relatively rare, having more durable components helps.
4) Shipping is sometimes very expensive and depending on the country, things can get lost in customs or be difficult to get through unless you know local language and bureaucracy well or have someone to courier or someone local who knows the system well.

Based on all that, I've toured on 700c in developing world and would so again, particularly if I have a bike that works well. My three part approach for failures in rough order is: (1) make sure equipment is durable and well set up before I leave, carry an extra tire or two if needed (2) if failure happens I can't fix locally, get myself and bicycle to a larger city and see if I can solve it there (3) worst case, plan on flying back from larger city back to US and bringing back what I need.