A ball drive at FRC scales has indeed been done before:
The omni-tread idea has not been attempted in FRC (as far as I am aware). There would be many challenges to overcome in manufacturing these types of treads. There are also other types of more traditional drivetrains which offer omnidirectional movement that would be far easier to construct. Still, the concept does indeed work at large scales:
http://www.youtube.com/watch?v=gpcv_-ICcIk