no there isn't dan (the i music etc shares the same brakes as the twingo gt) and in turn these share the same brakes with the clio williams and a standard clio range near enough with a few exceptions. it's easy enough to just get a hold of a rear hub for one of the above said cars, as well as calipers and discs. the install of said parts is within the reach of any mechanic. the only trouble you'll run into is the ABS on the rear and the handbrake cable.
just so you know, rear discs won't actually increase your braking efficiency as they don't do any of the braking (around 25% of the cars braking) so i am guessing this is for looks over anything ??