I have though about having a pause menu to select special attack, but I like the simplicity of one-button selection.
Seeing that we have a lot of stop and game play at the moment in the game, I might revisit that option.
"All" the button combos are already in use, I guess:
Up/down = Determines the slash direction when slashing (and you press that first before pressing the slash button)
A = slash (rapid-slashes when held)
B = jump (jump height is determined by holding it)
C = Special Attack (need to be instant, since it's used as a defensive way-out-attack a lot of times as well)
And you can slash when jumping so that rules out A+B as well. And A+C is impossible to press on a standard pad as well as any combination with button+Start, right? You need to change the attack quite instantly as times as well, so it needs to be easily accessed.
One a 6 pad, I now have change-special on Y and special-attack on X. You just need to move the thumb very little to access all of the controls.
To be honest, the more I think about it the lesser the pause functionality is needed. Unless you're in a boss fight, it takes a second to clear the screen of enemies then you can just leave the main char standing there. And if you have a 6 button one you have the pause as well as buttons for everything needed closely to each other, a good reason to be buying one.