I am also no lawyer, but I have to disagree with this: "the organization is going to be a lot safer if it can claim to be hosting only fictional works. By which I mean no manips or vids (copyright issues are less clear-cut)"

I've studied Art History, and there's a lot of legal precedent for artists being allowed to use copyrighted material, provided they change it enough to create a new work of art. (The most famous case would be Andy Warhol vs. Campbells Soup.) Manippers and vidders would be classed as digital artists, and so could use those legal precedents. (Which status, incidentally, could also cover adult manips against anti-porn laws: it's not porn if it's Art.)