I called the French Amex Plat Travel Service today and they said they said they had never heard of "centurion lounges". They put me on hold to check with the Centurion Service to end up telling me that such lounges don't exist and that I was probably mistaken.

I am however 99.99% sure that there is a "Salon Centurion" in GRU (at least last time I was there i.e. last March). The link provided by Brobbel, even it is out of date, prooves such lounges exist.

Here's another link for the sake of it:
http://www.americanexpress.com.bh/me...ts_airport.jsp

Apparently, showing a Plat card is enough to have access with a guest which would mean an additional Plat card holder could use this at no extra cost.