Well i was looking for a way to have pillars rise up in my mob arena. Then the creepers would be released and it would be like a cat/mouse game. I solved problem by just covering the bottom of the room in water.
There's a chance that a creeper might blow up the mechanism building the wall. Clearing the path for the next creepers.
Nope. That is impossible. Creepers cannot blow up water or lava. If the water source blocks are high enough, and the wall is high enough as well, there is absolutely zero chance for a creeper to blow up the 'mechanism.'
Then again there might be another auto-regeneration cobblestone wall, I've only ever seen one design that was pretty well done. It consisted of three layers- the inside wall, a second layer of lava, and a cobblestone outside wall. When an outside cobble block was destroyed, water flowing from the top would instantly regenerate a piece of cobble in its place.
OP: Build an endstone wall, and layer the outside of it with water. Completely creeper-proof. Water will stop all explosions. I know it's not an answer to the 'pistonable' question, but whatever.
There is no creeper-proof pistonable block. End stone is your best bet.
well i did some testing and i found that emerald ore is pretty good at being tnt resistant (it will only blow up if the tnt is next to it) and is pushable. i use this for my tnt cannon cause i needed it for the emergency switch (pistons with water). so ya try it!