This would be awesome! It is definitely not the greatest user experience to have to keep copy/pasting the text into a notepad file in order to save a long post while I work on it. Being able to save and come back later would be so useful!
I just featured a link to your post in my list of proposals to enhance the Steemit website :)
https://steemit.com/ui/@timcliff/let-s-make-steemit-better-comprehensive-list-of-suggested-ui-ux-improvements-to-make-to-the-steemit-com-website