moveable solids are hard to code, i know but what if you don't draw them, and they are a fixed size. You would drag them out of a menu. Download souptoys to see what I mean. I know this is confusing.
It wouldn't even be that hard to code. The issue would be how it would interact with other particles, which would slow the game down.
Cracker even coded an element that essentially was a moving solid, but the amount of calculation required to do its own collision detection would have been insane. If the way the slug in the ancient Falling Sand game destroyed everything it touched was any indication, it isn't likely to happen in this game anytime soon.
...If it does, well, the more surprise for anyone.