My concern with "famous" characters has nothing to do with their fame. It's the fact that stories that are adaptations still have copyrights involved in the stories, because they're derived works from an original. While it's always legal to post content that lapsed into the public domain, even if it's a derived work (like movies based on books), there's a greater possibility that the story has its own copyright, independent of the book, which is a search by owner and so much more difficult.
John Carter strikes me as a highly unlikely case, since the books that are still under copyright (and so could drive some company to decide it's worth taking the issue to court) are so much less popular than the early novels that are in the public domain, that I can't imagine there being a problem.
Well, unless they're newspaper strip reprints. Remember, each strip has its own copyright to check, possibly under different names, which is the ugliest search around...
