Partially correct: BA has stopped flying GRU-EZE but Lan Argentina still does a couple of times a day, though not every day of the week. Several LA-coded flights are codeshares with TAM which cannot be used for Oneworld RTWs, but the LA 4xxx flights are operated by Lan Argentina. Currently it seems they are only flying A320s on the route in which the highest class is Premium Economy. They used to fly 763s but no longer, unfortunately.

Mindpearl in Capetown is good for issuing ex-CPT/JNB RTW tickets; google for contact info, though I believe they are listed on aa.com as AA agents also.
Just found it:
Cape Town

Mindpearl
Mindpearl House
West Quay Road
V&A Waterfront
Cape Town 8001
South Africa
Phone 002721-440 6440
Fax 0027 21 440 6450 Monday - Friday
9:00 a.m. - 5:00 p.m.
(Except Public Holidays)

Happy Travels.