Ookla,
Copyright gets sticky in this area. First off, Mistborn isn't trademarked--you don't tend to buy any trademarks for books unless they get very, very popular. Though if someone tried to use it and produce things based on my work for large commercial profit, then I might have grounds for a legal battle.
The derivative works clause also starts to lose power when you change mediums. For instance, someone is allowed to do a drawing based on Lord of the Rings and sell it as their own work, with no copyright problems. In fact, you can DRAW the movie poster exactly, and then sell it, and it's considered your own work.
Copyright is a lot less clear-cut than sometimes people like to think. Mostly, it depends on how the judge feels about whether or not the new work is cutting into profits of the original.
So, if I were producing my own Mistcloaks, and I wanted to bay the ridiculous amount of money it took to register the Mistcloak trademark, then I could PROBABLY sue that company in China. However, they could just change the name of theirs to "Mistgoer's Cloak" or something like that and be just fine. They can be inspired by my work and create a product without officially infringing upon me. Just like someone could read Harry Potter, produce and sell 'Wizard's Wands' that look like the ones described, and be in no trouble at all--as long as they didn't call them Harry Potter wands. (And that's only because Harry Potter is trademarked.)
The long and short of this is that I--as I understand it--couldn't stop Nessa even if I wanted to. She's not using anything trademarked, she's not competing with me, and she's not claiming to have official backing.