I can not verify the historical accuracy of a kilted pirate..... However, aboard ship it would be impractical to wear a kilt. The apron mentioned earlier has been documented in several books as have the breeches. I don't, however, see why a good Scotch Pirate wouldn't wear his kilt on dry land.
Now. As a member of the SCA I see kilted pirates quite a bit at events, especialy Highland war.