It helps that batteries have literally two orders of magnitude greater range than an old school flywheel.

Yes, definitely, they are trialing this in Arnhem, I'll go have a look at it to see how it is all put together. By the looks of it they take a 'normal' trolley bus, remove diesel engine + tank + associated bits and pieces then put a battery in the spot where the tank used to be, and a bunch of electronics (regulator/inverter/charger) to create a voltage high enough to run the normal bus hardware of and to recharge the battery when the bus is connected to the overhead grid.