Stickman is a moving solid. :P
So is wheel, slug, -anything- that moves and propagates that maintains itself distinctly as a whole of several parts.
The only difference is with stickman, is that it only collides with particles. It isn't -made- of them.
If you want a literal ball element, just add gravity to electrons/neutrons, and they function essentially the same way without the element identity copying. Any ball element in Powder Toy would basically be just like Stickman, in effect. Not hard to do at all, it just probably won't be as awesome :P
Basically, all I wanted was a bouncy particle that can take on other's properties.