Red: I'm a little confused by the question. Are you saying that the tartan you're thinking of is the personal tartan of someone with the same last name as you? If that tartan is restricted to that person, I doubt you'll be able to get your hands on it. As such, if you can get your hands on it, it is probably not registered and, therefore, fair game.

All that said, on this site you will hear again and again, wear what you wish so long as you always treat every tartan with dignity and respect. It is even better if you know something of the tartan so that, should anyone ask about it, you have more to say than "I like the colours."

That would be my thoughts, unless I've misunderstood your meaning.