Interesting thing I read in some car magazine recently (Motor Trend maybe) about directional tires, which most/all high performance cars have. The directional rating is for the tread, not for the tire, so the tire can be run backwards, but the tread will not behave the same way, water channeling, hell it'd probably grab water to put under your tire. The thing I read mentioned how in places like Phoenix, LA, and such people with Vette's, Porsche's, etc, would routinely run them backwards to get more life out of the tire, on dry pavement there was no problem with running them backwards, the tire can take it, just the tread is designed to only handle water from one direction. Might be louder too backwards.
So, point is, you can certainly run a directional tire backwards, infact several rock crawling guys would do that on purpose just to either be different or having it backwards was a different tread lineup (not flowing water, just looking for edges to grab).
I have MTRs on mine which are not directional anyway, so it hasn't been an issue. My front tires get cupped a bit, I try to rotate the spare in and have every tire going a new direction when I rotate, it rides a bit rough for a day or so while the new pattern is broken in, but then is fine.
Good Luck,
Mark Brodis