(defun buffer-out ()
  (interactive)
  (let ((buffer (current-buffer))
    (value (buffer-string)))
    (kill-buffer buffer)
    (setq s value)))

(defun edit-string (string)
  (let ((buffer "*edit-string*"))
    (get-buffer-create buffer)
    (switch-to-buffer buffer)
    (set-buffer buffer)
    (let ((inhibit-read-only nil))
      (insert string)
      (fundamental-mode)
      (local-set-key (kbd "C-c C-c") 'buffer-out))))

(setq ok (edit-string s))