Even if physical publishing is not a concern here, fanfiction by itself carries a stigma, even among many of those same 12-30-year-olds that would be its target audience. And it can negatively reflect on the author and the community by association if said fanfiction is given excessive emphasis.
“Yudkowsky? Is that the one who’s writing an insanely long Harry Potter fanfic?” “Well ummmm...”
More description would be nice—right now it’s quite vague.