Ahpiox:
Why not?
If you mean this as in the metal ball acting as one object, rather than a bunch of individual pixels, then it would probably qualify as a moving solid.
Problem is, TPT is coded in such a way that each pixel kinda does its own thing; you can't make multi-pixel structures that stick together. As fun as it would be, it's just not possible without a huge overhaul
EDIT: Ninja'd
Moving solids have been rejected, because they cause lag and etc. For example jacob1's mod proves that they work half the time.