Then there's the kind of bas relief sculpting that can be done with abrasive media, like what is done with "sand-carved" wooden signs.

There's really no reason an accordion body couldn't look like Kermit the Frog made out of urethane foam, as long as there was a place for bellows in the middle and buttons at either end.

And why not buttons in a curve like some computer keyboards? You could cast an impression of the heel of your thumb to get the proper shape for the back of the button board...er-ga-cordionomics

All of the methods mentioned could be used for decorating accordions. The problem is return on investment!!!