If you're dates aren't quite flexible then I know its a bit further out of the way but I expect you're going to find availability on ET much easier than SA in C.

As the previous poster mentioned UA Mileage Plus is probably the way to go (no fuel surcharges on award tickets) but depends on your UR balance. Applying for one of the US based UA cards could get you there too. Unfortunately SPG to UA is a poor ratio and it's not an AMEX MR partner.

I'm not as up to date on Oneworld but isn't/wasn't there an MH flight JNB to South America as well?