After reading several articles about flywheels it appears that they are best used for providing acceleration rather than constant power. It has to do with how efficiently energy is first transferred to and from the flywheel vs the equivalent amount to/from a battery. The ideal hybrid system would have a flywheel the provide accelerative power and a battery for constant running. This article sums it up pretty nicely:
http://www.hybridcars.com/related-te...l-hybrids.html
This doesn't look like something a backyard mechanic could safely tackle. The forces involved are huge and dangerous if sufficient safety precautions aren't employed. Back in the '90s Chrysler tried, and failed, to build a hybrid flywheel race car. Just google "Chrysler Patriot".