First, it might be hard to find anyone who will officially say its ok to run a directional tire in the opposite direction due to liability reasons.
With that being said, from a practical viewpoint, a modern radial tire does not care which direction it rotates (This was not true of older bias-ply tires). The tire will not shred or fail just because its run in the wrong direction.
However, there are a few likely downsides to this practice: (1) Running the tire backwards in the rain is a concern; (2) Some tire tread will be noisier when run backward; (3) Cost. As noted above, the cost savings might not pan out when you consider the cost to swap and rebalance.
Here is good article on the TireRack site for reference:
You're going the wrong way! (RIP John Hughes) - Hunter's Ramblings about Performance wheels & tire | Tire Rack