When you break an Redstone Powered Piston you get it back in Creative

I believe this works for any thing that occupies two blocks (doors, beds). If Mojang fixes this, I'd like to see minecarts not drop themselves as items either. 😛

There is a block state called "half" and the second block that the piston uses is called the "piston head" with a separate ID and a block state.

There's no "half" block states on pistons, there's 4 IDs: piston, piston_head, moving_piston and sticky_piston; all have a facing state, head has a sticky bool, piston and sticky piston have an extended bool and the head also has a short bool (which just removes 2 pixels from the model).
The "progress" is NBT on the moving piston block.