I think it really depends on how hard LJ wants to push it and how seriously the actual owners of the propritary products feel. Anne Rice, Anne McCaffrey, for example have always cracked down hard on fanficcers and fanartists etc. Its always been illegal, so LJ can easily say NO.
On the other hand... most of the people who have a legal right to stamp out fanfic don't. The Anne Rice and Anne McCaffreys are fairly rare. So does LJ want to stamp out Star Trek fanfic when the show producers have openly condoned it for years?