Click HereYou can add flex to any weapon model node as normal. However, when working on a mace with a cloth "seal" on the grip for someone's project (here's an
image of it) I realised that item models never actually move much in NWN - they are swung around by characters, but their actual position relative to the character's hand doesn't change, and so danglymesh nodes on weapons tend not to move as much as you'd expect. Wind will still affect them, though, so in general a node will look more natural than if it didn't have flex.
Another option is to animate a part of your weapon (using the same anim slots as PCs; check the whip "m" part for an example), but that's obviously a lot more work.