It requires writing some assembly, actually. Somewhere in damage calc there's a check to see if the attacker is holding an item with the Choice Band effect. What I did was make it so that, instead of directly increasing Attack, it branches to my own method, which checks the item again and increases the appropriate attacking stat depending on which Choice item it actually is.
There's not a whole lot I can say that would help with this other than encouraging you to dive into the ASM and try to figure it out yourself :/