Thanks for the responses everyone, much appreciated!
To answer your questions, I have a French passport and a valid ESTA so I don't need a visa for Brazil and wouldn't have any issue spending a night in the US either visa-wise.
I've checked one-way and round trip tickets and one-way is around 2 third of the price of a round trip so I'm better off buying a one-way ticket than buying a round trip one and losing the return ticket.
AA_EXP09, if you want to meet in person in Hong Kong we could totally do that but I'm helping a friend of mine move on Sunday so might not be available that day... How long will you be in the city for?
I'll look at all the options you've given me and see if I can find anything cheaper than what I had already found. Thanks again guys!