Deterministic method to get the Fairy Flute
Okay, this only works on emulators with save states. And if you're playing like this, you're probably using Moguri Mod.
But that's what I found: the RNG pointer moves every so often. It just happens that this interval is one arm movement of Hilgigars.
So the script goes:
Pre-requisite: make ATB Wait
Make sure that Zidane will go immediately
Enter the steal menu, save state.
Confirm, see what he gets.
If it's not the fairy flute, load state, let Hilgigars move arm once, confirm.
If it's not the fairy flute, load state, count two arm movements.
Repeat until you get it.
I think this is useless for most people, but I found it neat that it works, and also answers a question I had about the RNG pointer: yes, it moves with time, not only actions, during battle.