Legal advice I cannot give. But gut-feelings are easy and free. If you ask me, I think you're worrying a bit too much about this. Somehow I strongly doubt there are hordes of would-be thieves just waiting for the moment you post up your design to pounce on it, beat you to having it registered and woven, and you missing out on riches and fame.

(Sorry. I wasn't trying to come across as snarky. I really wasn't. But I am relying on a little bit of hyperbole here)...

The fact of the matter is, most people design a tartan that is, above all, meaningful to them... And it's something that they, themselves can be proud of because it's something that THEY designed and created. There is nothing to win or gain by stealing someone else's design. Designing a tartan is for the love of the thing, not because there is a sense that one will profit from it somehow (not usually, anyway).

So my two bits on this -- take a deep breath, relax, design to your heart's content, and feel free to share with the rabble, your family and your friends. If I'm wrong, and all of a sudden there's a Chinese sweatshop churning out Morris at Heathfield knockoff tartans... I'll eat my kilt!

But if you are simply asking for the sake of curiosity and "what-if" as a thinking exercise, I really don't know how trademarks work when it comes to tartans.