For private submissions, I'm planning to remove the character limit on titles. (The idea is that users should be able to submit whatever links they want without worrying about titles or tags. It's just a bookmark, and a place for you to take notes.)
If I enable tag editing, that introduces some strange cases: someone could submit a link with a big title, then switch it from /l/private to something else.
I could add logic to prevent people from making a private submission public, and vice-versa. But it's easier for now to just submit a comment asking to edit a tag. (Or email [email protected])