Just beause an offense is a spread doesn't mean RB's automatically don't get lots of yards. RichRod had several 1,000 rushing seasons from RBs at WVU. Not all spreads are created equal. The RichRod spread is vastly different from the Meyer spread, which is vastly different from the Okst spread, which is vastly different from the Oregon spread, etc etc.
Point is, Meyer has never had a 1,000 yard rusher because he uses the RB differently from how Michigan has used the RB. And besides, Dunn may not seriously see the field until we have Gardner/Morris as our QB, at which point he'd be used in an offense that is closer to pro-style anyways.