Two comments:

1. If people really want to have a "Clan X" you don't need to worry about "registering" a crest badge and motto. In the Real World (tm) that stuff is handled by the Court of the Lord Lyon in Scotland, but since we are not talking about an actual clan, but more of a "club" then you guys can design and use whatever kind of crest or logo you wanted to. Hank's already made up some XMarks kilt pins, right? Why not use that as a crest, that's what they were made for.

2. On the tartan (once a design is completed, of course) when people are ready to have kilts made, just ask your kilt maker. Any maker of traditional kilts will be familiar with the woolen mills and will be able to handle that aspect for you. You don't need to worry about finding a mill and all that. Just ask your kilt maker!

Aye,
Matt

(BTW, if I were to design an X-Marks tartan, I'd use this board for color inspiration. That means blue, white, and grey -- and maybe a bit of yellow for the emoticons!)