Interesting idea, but it probably doesn't need to be that complicated. I think the little rhyme is nearly literal anyway. The Finns specifically ask Mat if he has no items to make a fire, iron weapons, or musical instruments. The iron part is something RJ likely took out of folklore, various creatures like fairies and such are harmed by the touch of iron, and the whole need for the bronze dagger to enter the tower might be related to this. Music might have the power to mesmerize them or something. In any case, I believe Mat's daggers, Aludra's matches, and Thom's harp will be the weapons used against the Finns.