Considerate non-member person wearing a clan or family's tartan, there should be no problem from any associated person, IMHO. Wearing one to blow one's nose in- now that is another question, even by implication. However since such use of pocket squares is so very rare in North America these days, not likely to come up as an issue here.

I do have some very nice tartan hankies made by Barbour in their company's tartan designs, purchased at the US outdoor company starting with the letter O that I lately seem to over-advertise on this site. That is a nice compromise but they were no bargain either. More along the lines of an extravagant impulse buy.