I don't see how this would work unless it's stored in the browser's local storage, and then it's only on the machine you wrote it on.
The reason I think this is because otherwise it'd be saved to the blockchain and anyone could just wait for drafts to be saved and then copy them.
You could encrypt the post first before saving it, but this adds a whole layer of hassle to something that can be solved by saving it to a file yourself.
Perhaps I'm overthinking it though.
You could very well be right. I hadn't thought on the implications of the blockchain. Still even if it could be saved locally but you were still within the steemit app that might be a help.
Perhaps I was underthinking it! :O)