What could be done is to give all the bandwidth that is left from local broadcasters to the cell phone companies in exchange for broadcasting the tv stations on all the cell towers in the DMA. This would guarantee full utilization of all the bandwidth available that the local broadcasters are not using. Of course, white spaces is supposed to be doing this with wifi already. They would not be able to transmit the same frequency on as many towers to prevent interference but they could use the same tower that is broadcasting tv for broadcasting cellular service if they ain't already.
What's to stop the local stations from offering their own wifi over the unused broadcast spectrum using it for a subscription internet service or ad supported service or even to broadcast their own tv stations.