Well, it's often been said that you don't have to be Scottish to wear a kilt, anymore than you have to be a cowboy to wear blue jeans. There are numerous universal tartans that one can wear without fear of rebuke.

Plus, many hold the idea that wearing the tartan does not necessarily mean to imply that you are claiming membership in that clan. Only if they explicitly claim to be a member of a clan, and aren't, well, then they're a bit "Walter Mitty"ish. But it's like wearing a sports jersey. Wearing it doesn't mean to everyone who sees you that you're a professional sportsman; it just means you're a fan. So it seems that you could be able to wear a clan tartan as a "fan" of sorts. Just have the courtesy to know what clan tartan you're wearing and perhaps some of it's history, and you should be okay.

As well, any military connection is allowed without question. I was a member of the Seaforth Highlanders of Canada, and I would be surprised - no shocked, really - if any member of the MacKenzie clan had the audacity to question my right to wear that tartan. Plus, I was also a member of 2 RCR for a while, and that battalion was created out of an amalgamation of the 1st and 3rd battalions of the Black Watch and some Guards regiments in the early '70's. So, even though that tartan is universal, I wear it knowing I have connection to it (albeit convoluted).

So, my not-so-humble opinion is that you should be able to wear whatever you want (except the privately owned and protected patterns {like the balmoral tartan} - but don't worry to much, reputable kilt dealers won't even sell them to you, so you're unlikely to accidentally get one of those). Just, if you're wearing a clan tartan, and you're not actually a member of said clan, admit it, and say that you're a "fan" of the tartan - that's all.