IMO the real difficulty is manufacturing anything besides a sphere that has better properties.
Bubbles are spherical for a reason. It is the lowest energy state of the liquid - another way to think about it is that you've minimized the surface area to volume ratio. The material is as relaxed as it can get, given the constraints of internal volume and surface tension.
When paintballs come out of the encapsulators, they're really, really soft. Spheres are their "natural" shape, if they could be made to float while they dry (which is basically what the tumblers do) they'd be very spherical. See 3:35 in the following video.
Without spending a ton of time and money investigating it, I don't have much to contribute. The barrier for me is I don't know enough about potential manufacturing processes.