Been looking for a work around on this myself and think I have found one.
I created a blank plain text document called "untitled text" and saved it to my desktop. I dropped it in to the outbox.mbox folder and renamed it "untitled text.emlx".
This seems to force the outbox to show when you open mail, but mail doesn't actually read the document as an email so the mailbox appears empty in mail.
This means you don't get the annoying "cannot send this email" message pop up every time you open mail or when you click "get mail", that you do using the above options.