Yesterday I experimented with using Ellipsus's ao3 export feature. I copypasted what I typed up in Scrivner and then exported to ao3. It will take a while to find your account, but once it does, it's pretty easy. I still need to see how it deals with horizontal bars since I find that's something ao3 hates having copy pasted in if you aren't doing HTML, but otherwise, it went nicely.