The rules really fluctuate based on whatever Laurel is feeling at the moment. It's not written anywhere (except in those occasional rejection letters) that there are any rules. It annoys a great deal of us myself included.
However as you saw the "rule" is that you can't right non-con with celebs/copyrighted characters. Star Wars is probably maybe possibly safe as long as you're only working with original characters. Again this ultimately comes down to what Laurel/the mods are feeling when they read your story and trying to make rhyme or reason out of it will just make your head hurt because you might very well have got a hard non-con story with Supergirl and Lex Luthor through and then had a black mail story get shot down later.
MY adice though is to use an expy. Have a story about Supergirl? Call her Ultragirl or Athena (Gods Greek or otherwise can't be copyrighted) who came to save the world blah blah blah. You know just like porn did for years before the parody laws were rewritten (or smarter lawyers showed up I honestly don't know which) but there is no rule against Muffy the Vampire Slayer being enslaved whomever.
PS: This isn't a Story Idea, it's a question. It will likely get moved to an appropriate forum. Authors Hangout would likely be a more appropriate section for this.