Personally, I would avoid staying in Zona Sul, unless you fancy commuting for 1-2h each way.
I might be tempted to stay around Riocentro where there are lots of high-end, new hotels (Grand Mercure, Hilton) within easy reach of the conference. Once you’ve relaxed at the end of the work day, you can head out at 8pm to eat and drink without any traffic (you can hit the bars and restaurants in Barra or even Leblon (20 and 40 min respectively without traffic); on the days you feel lazy you can hit a mall for a meal and people watching). Budget-permitting, you can stay at the Hyatt which is by the beach, but near enough Riocentro.