Can’t really help I’m afraid, but there is definitely a pretty visible hipster-ish fixie scene, and some mega expensive road bikes being ridden round the posh parts of town at weekends. This is based on one fleeting trip to the city so you maybe know this already. I was also chatting to some guys on mtbs in the Jardins area who were taking a huge group of newbies around. They were pretty enthusiastic but I can’t recall any details that might help. I’d imagine that there would be a local riding scene amongst the upper middle class (who seem to be absolutely minted beyond belief and just piss about with expensive toys half the time) and that they’d be pretty easy to find online if your portuguese is up to it. Whether the riding would be up to much is another matter.
I assume you’ve been if you’re considering a move there. Pretty mental place (in both good and bad ways) in my limited experience, and kind of defies comprehension in its scale, complexity, inequalities and so on. Like many Brazilian cities I suppose, only seemingly more extreme (if less immediately apparent than somewhere like Rio perhaps). Before I’d been almost everyone (gringo or Brazilian) who I met who had been was pretty negative about the place but I don’t think that’s entirely the fault of the city itself – more reflective of their attitudes and prejudices. Having said all that, I’d be curious to see what the mtb scene is like…