Honda's hybrid system is also very different, they incorporate the electric motor directly mounted onto the main engine. The motor is smaller and lighter. Toyota has the motor completely seperate which adds more weight.
Really the added weight is the battery pack and the electric motor. Depending on the size of the pack, those could weigh alot. The tesla S battery back is massive because it's all electric, the car is the size of a camry but weighs an extremely hefty 4,600lbs due mostly to the battery.
70KWH battery pack weighs 1200lbs and the 42kwh pack weighs about 825lbs for comparison.
At roughly 18lbs per KWH, the prius plug in has a 4.4kWH pack so that's about 80lbs for the battery pack in a hybrid. Add in the weight of the electric motor, wiring and a few other components, i don't think a target weight of 200lbs would be unreasonable. They make electric motors that can be modified to fit between the transmission and engine, or on the back of the transmission. Perhaps they can attach it to the rear end. I also don't think you would lost much efficiency if the system used the VSC to help modulate braking power to the rear when it can, like slowing to a stop at a stoplight.