Greetings Mack,
Welcome to the FRA forum, always glad to see our friends from across the pond here!
You are quite right, there is no limit to the KE that can be stored in a flywheel, used as a weapon or not. (rule 11.2 & 11.3 applies) What kind of application for a flywheel do you have in mind? If I knew this, maybe I may be able to offer you some more detailed advice.
Regards,
Geoff.