I just booked such travel myself. I tried Virgin, Delta, Continental and Air France to book the travel. I found Virgin and Continental to be the best desks to book through. I would have gone Virgin all the way (LAX-LHR-JNB) but I prefered to stop-over in Paris and balked at the 200k miles for Upper Class that Virgin charges.

Instead, I booked LAX-CDG-JNB on AF in Business Class with a 5 night stop-over in Paris and then JNB-LHR-LAX in Upper Class with a 6 hour lay-over. I took a longer lay-over in London so that we could take the tube into the city and stretch our legs before another 11 hour flight.

The AF/VS booking cost 150,000 miles through Continental as they (for some reason) charge 20k less miles for a RT in Upper Class to SA and AF Business Class is substantially fewer miles than Upper Class.