As best I can tell, the "itchiness" of wool is a factor of quality. The more expensive wools don't itch, while the cheap ones do. When I was a kid, I had a wool sweater that I hated cause it would itch. I was surprised to learn my Air Force uniform was wool, cause it never bothered me. And my tank, that my parents dropped an unknown large sum of money on, doesn't bother me either.

I have some historical repro' kilts that are made with "affordable" wool, and those will bother me if my skin gets the least bit moist. Luckily I wear a historically accurate shirt with that wool, so I have little to worry about in my "lowlands" as such shirts are very long and act as underwear.

And to echo what has been said already, I know of no way to reduce the itch-factor of wool once it is off the sheep and woven. Perhaps some of the professional kilt-makers in this forum can elaborate further.