Thanks for the replies, one and all.
Will probably take a walk around Copa beforehand to see if anywhere looks lively, and will drop by this Blue Agave place for a look. Be good to find somewhere with a bit of atmosphere - even the commentary in Portuguese would be a start! I've been to Rio quite a few times - I live in São Paulo - so I know Lapa tends to be quiet until the evening.
As for the derby, I'm a São Paulo fan over here so I'm just going for the spectacle and will 'supporting' whichever team's fans I end up in the middle of!