(Someday, I will have to watch some SGA so I can read all the nifty fic that gets thrown around.)
I think the problem isn't so much "what is chan"--where we know we've all got slightly different definitions--but "whose def of chan do we warn for?" Do we warn broadly, and risk losing readers who are perfectly willing to read about 15-year-olds tearing each others clothes off, or warn narrowly, and risk squicking or triggering people who can't handle a 22-year-old college student seducing her 14-year-old brother's best friend?
In the case of the OTW, much of the debate is academic: there will be plenty of tags, including author-created tags, available; you can warn for "implied chan." But for those authors who want to put the minimal amount of info or "spoilers" on their stories--it's hard to figure out what is, and is not, "chan" in the borderline cases.