It may just be a matter of semantics but let's assume that a tubular undergarment worn under the kilt is a liner rather than a slip. I was under the impressions that the garment that women wear is called a slip because it allowed the outer clothing to keep from riding up...a kilt liner would be worn to either keep the kiltee extra warm or to - shall we say - reduce the abrasive action of the kilt material upon one's nether region. If someone were having problems as a result of contact with wool, I'd say that it was totally fair to wear some kind of a liner.

Best

AA