I think you could actually make one without too much fuss. There's a tool called Touch Portal that can turn an Android device into a macro keypad, and it doesn't have to be physically connected to the computer, so there's nothing in the way of getting a small phone and making a strap for it.
@foone Wait... like this? Because I want this too. And the fact that these never went back in to production is proof that Nintendo has lost their cool.