Thank you for all the replies.

My own instinct, which I kept quiet about in the original post to encourage discussion, is that registering our own corporate tartan is the best way to avoid cultural appropriation.

I've always felt very uncomfortable with the idea of wearing a tartan from a clan or region with which I have no connection, so I tend to stick to Irish-American / Celtic Heritage or universal tartans. (Typical Irish family -- we have no idea where we came from, and the surnames are entirely too common to be of any help. And to make matters worse, 23 & Me revealed that I'm actually only about 1/8 Irish despite having been raised half Irish my whole life. Apparently marrying an Irishman made you Irish even if you were 100% English or German.) I've recently solved the problem altogether by registering my own tartan and I'm now saving my pennies to get it produced. The kilt, to me, is not distinctly enough Scottish or Irish for me to feel like I'm being appropriative wearing one as long as I'm wearing it in an appropriate tartan. So creating an organizational tartan would allow those who just love kilts to finally have a "clan" to identify with, albeit a corporate one.

But, again, those doing the appropriation never feel they're being inappropriate, so I want more perspectives on this.