Hmmm Rio in the box in 15 minutes...
The main problem with camping Rio (at either end really), is that typically it takes a while for the units to trickle towards you. Granted, that is partly the point of the tactic with regards to completing games, but it is relatively counter productive for faster times.
Not sure what composition would be great for this, probably Demo, Valkyrie, Jugg and maybe GTS with some Reegar IA action thrown in. At least if you are talking near pure box camping.
If box is mainly last resort, then you could throw in a Javelin character I suppose.
In any case, it is the objectives that suck on Rio. Jugg partially exacerbates this since it takes too long to get to objectives at the other side of the map, so hack or escort progress is going to be slower than it should. Hack in the "power station" near the box all 3 times wouldn't be too horrible time wise I guess. Targets potentially come to you, but potentially require tracking them down depending on spawns (so you don't go below the time limit).
From my experience in Rio pugs from way back when this map was practically the only platinum map you landed in outside private lobbies, the issue is most of the players have a lot of difficulty doing the cross map escorts, some hacks, and devices without dropping. Cross map escort is very difficult to hero/solo, since the movement speed is so low and the railing isn't great cover.
And finally, gold pugs are fairly unlikely to actually be 15 minute games. Rio platinum pug in 15 minutes just seems like a tall order.
In any case, if someone wants to play Rio, it doesn't bother me. I would recommend a different map if they asked for my advice though.