But for a term to have any value, it has to have some boundaries.

It does have boundaries, just as Flotsam has boundaries; the boundary is context.

If some of my buddies and I showed up at your club with electric instruments and performed Slayer's 'Reign in Blood' in its entirety, would that be Folk Music?

Personally I feel a bunch of guys doing cover versions of heavy metal is Folk Music by default, but would it be worth the effort of setting up all your gear just for a couple of songs in a floorspot? One things clear, from past evidence you'd go down a storm at Fleetwood.