There is something engineered like that, they used it in de f1 kers.
And there bussy working on a street hybrid one, called a flybrid.
Volvo is going to use it and Jauguar has also a prototype.
I really like the idea, your not converting power but simply storing kenetic energy. Thus getting a claimed efficiency off 85% !!(Electic reg. braking 30%)
And the weight and enviremental impact are also far less.
Williams Hybrid Power - Flywheel Background
Gyrobus - Wikipedia, the free encyclopedia
Volvo touts mechanical KERS for future road cars (but fails to mention partners Flybrid & Torotrak)