What teams have desgined a 6 wheel drive system?

[Madison]

I've always imagined and wanted the opportunity to design and build, for anyone who's interesested (hint, hint, nudge, nudge), a six-wheel drivetrain that keeps them all at a single height, but uses omniwheels on the four corners. It seems like that's a simple solution for enhancing traction in the robot's direction of travel while minimizing additional friction in turning.

I'd be surprised if another team has not yet tried this method, though nothing springs to mind.