Right clicking an armor stand with a stick in hand consumes the stick, and gives it arms. In order to take the arms, mine the stand, and it will also drop the stick with itself
+1I really want my armor stands to have arms, especially so the armor stand can hold the full(ish) 100k set.
It's a great idea but I'm not sure this will happen. You have to use commands to summon armor stands with arms and to make them hold things. It would be too easy for someone to find a bug and duplicate promos.
Command blocks aren't at all a necessity. All they do is set special nbt data, which is easily done with plug-ins as seen with bosses, custom items, custom promo mobs, etc.