Actually, a large number of the Penicuik sketches show left handed highlanders, though there is some debate as to whether it was to make them appear exotic, or simply the artists' fancy. As a student of Highland Swordsmanship however, their postures and guards are correct to Thomas Page's 1746 manual, so they can't be completely based in fantasy...

And in regards to the dirk placement, there's much iconography for it to be worn in the centre of the belt - accessible to either hand, though these sources are certainly not for formal occasions.