Chris,
While I have no knowledge of this thing called "misting", generally you don't riff the published stuff for a very simple reason. It's much riskier legally. If I riff a Star Trek fanfic using Mike & the bots, and the fanfic writer gets mad, there's really not a lot of legal recourse. He can't sue me as he doesn't have legal rights to the Star Trek characters. (Well, he can try, but Paramount will probably become very interested very fast and sue him.) But if I do the same to a Star Trek novel, Paramount and Del Rey (who *do* have the legal rights for the characters as well as a possible plagiarism case) could not only sue me but also the folks at BBI. And that would really suck, especially since we'd be exposing people who had no idea as to what we were doing to a lawsuit that could forever doom a certain cowtown puppet show.
(Not a lawyer. Don't play one on TV. My legal advice may or may not bear any resemblance to the laws in your area. And yes, "Fair Use." Blah, blah, blah. I'd prefer that I, nor anyone I know, not be the test case for whether it applies or not for mistings. Which, of course, I've never done.)
Oh, and Gumba, they have done that to some webcomics. I won't mention one involving a certain artist with a liking for fursuits and a liking for also threatening lawsuits, but there is a rather well done one out there that riffs one of Jack Chick's tracts about the evils of D&D.