Hmmm ... no fireworks mode here (a570is), but... found some interesting functions, tried them out:
NHSTUB(PutInNdFilter, 0xFFE06164)
NHSTUB(PutOutNdFilter, 0xFFE061AC)
and added a new menu entry to test them... it works, but it only seems to close the aperture down to f/11, at least the Av value showing up indicated that
Well use would be very limited anyway, it seldom that you need even less light
Edit: Maybe the sound is a bit different because the aperture normally does not go that low (without hack)?