*nods* When MPAA got their knickers in that ridiculous twist about NC-17, I finally threw it all over and invented my own scale for my archive. Like you, I wound up with a combination of rating and labels to give my readers a reasonable guide, but my ratings went specifically for the "intensity". So I have I-1 through I-5 plus reasonably specific genre labels to say what major themes are deal with--Romance with Porn, frex.
I do wonder about about using Worksafe. It seems it might be best to leave worksafe and non-worksafe as their own continuum, with their own already established utility, and call the least explicit rating something else. Maybe even just Non-explicit, since it seems like you want to aim for a scale that can encompass all themes and simply address the level of down-and-dirty they're dealt with on.