G'day oz. The most logical solution that comes out at me is a return to Paris using either Aerolineas or Lan Chile. Both probably offer a return side trip in Sth Am to get you to Sao Paulo, but neither are in *.
I can't see any other Oz-Euro fares (as in normal special or published BNE-PAR returns) that allow journey via Sth Am.
A *RTW seems feasible, but then the glaring problem with that is being based in Paris, you wouldn't be able to use the *RTW as one stop only per city, so you can't keep coming back to Paris. One way to get around that is to get to Paris, then head to Dublin, come back to Brussels and train into PAR, train back to BRU to go to MUC then back to say AMS and train to and from PAR, then AMS to MAD etc. A lot of mucking around though with this not to mention train fares. If you get any * carrier or combination of * carriers normal return to PAR then you are eligible for a Euro airpass, but with the sectors and travelling involved, I guess this will not be cost effective.
The same goes for a more direct *RTW routing say via MIA - you would need to pick up a cheap web fare from there to SAO and back, but cost effectiveness again comes into play.
You could go over to the dark side and use ow as you will get what you need there. *Gasp* *Horror*
FWIW, a BNE-SYD-LAX-SAO-PAR-BKK-BNE routing for a *RTW is approx 30,100 mls.
Cheers/Regards.