No bike in Pakistan in these classes have a Rear Wheel Disc Brake. Only Front Wheel Disc laga k laaakh rupay maang rahay hain bike k liye :S
As fas as Modified Disc Brakes in CG-125 are concerned, i haven't seen A single person who modified a CG-125, Ravi Piaggio, or a Deluxe with Rear Wheel Disc. However, there was a person with a Completely Modified Suzuki GS150 who modified the whole bike to a Type R, painted it blue, and installed both Front & Real ALLOYs with Discs.
Also ... another CG-125 i saw at PakWheels whose Front Part (complete) was modified to a Kawasaki GTO's inorder to install Front Disc on it. Again, no rear wheel disc.
Piaggio & Deluxe are coming with Front Discs factory installed. Rear brakes are still Drum Brakes.
I hope this helps for what you asked.