The permanent fix - and yes, I rode the mountainous Cycle Oregon on a tire with a much bigger cut and my fix - dacron sailcloth and contrator;s grade contact cement.

Go to a sailmaker and ask for a scrap of dacron racing dingy jib cloth. Cut it nice and big. I sometimes go bead to bead and an inch more in both directions. Bond it in with good contractor's contact cement, the stuff will all kinds of bad solvents. (Take the warnings seriously and work in good venalation.) Done carefull, the tire will not bulge and you can ride it the rest of teh tread's life with no worry. Just don't fold the tire at the patch or you will find yourself needing to reglue it.

I spent a few years riding the Vittoria Open Paves, the green treaded high end rain tires that cut so easy. (I think the grippy tread is a magnet for road debris.) At $70 a pop, tossing them was killing my wallet. Once I started with these patches, I could ride the tires into the ground.

Ben