You can edit your posts in a private topic; most of us power/beta users use it as a notepad; simply don't add anyone to the discussion and you have yourself your own private galla of notes/thoughts.
You can save and edit your posts in the PT, then when you're done, edit the post and select the content and post it on the forum. It is a perfect answer to your request, as it will be a draft in every form, since it'll allow you to save it without anyone seeing it, and test formatting prior to posting.
