I think it would be difficult to claim copyright for a kilt and could create more problems than it solves.

Also many will see it as sour grapes against a business rival - I am sure that is not Howie's intention.

Attacking the misleading labelling and insisting upon country of origin and information upon the fibres would be a better tack. No doubt they would find some way of getting a Scottish reference in like "Scottish style kilt" or whatever but the rest of the information would remove any misunderstanding.