I think the chances of you finding someone who 1) transited internationally through GRU on TA 2) on a 12h layover, and 3) from a country whose citizens are normally required to get a visa to enter Brazil (i.e. therefore had to be especially attentive about the specific need to stay airside) are going to be slim to none.
So speculation is the best you can hope to get here, and the best people can offer you is that in theory it should be feasible but there's also a fair chance of a major SNAFU along the way.
If that's not good enough for you, my friend - and you can't be bothered to get a Brazilian visa just to be on the safe side - I recommend you take your own advice and fly to JNB direct from the US, or via Europe.
Last edited by RafKa; Mar 10, 2017 at 7:56 pm