I eagerly studied gff tutorials thoroughly and here's the first result:
Hattori Hanzo's Katana, inspired especially by the Crazy 88 scene from Kill Bill vol. 1.
This melee weapon is too powerful for serious playing, as it severaly disturbs the game balance, but it's meant for "after-playing-through-for-the-first-time" fun. That's the reason why this item can only be accessed through cheating (and maybe the fact that I didn't manage to modify scripts... ;) )
Without further ado, here's the file:
http://merko.phkk.fi/~dicdom/katana.zip)