replace-string-in-rectangle

(defun replace-string-in-rectangle (from-string to-string &optional delimited start end)
  (interactive
   (let ((common
	  (query-replace-read-args
	   "Replace string in rectangle" nil)))
     (list (nth 0 common) (nth 1 common) (nth 2 common)
	   (region-beginning)
	   (region-end))))
  (let ((replacement-rectangle
	 (mapcar '(lambda (arg) (replace-in-string arg from-string to-string)) 
		 (extract-rectangle start end))))
    (delete-rectangle start end)
    (goto-char start)
    (insert-rectangle replacement-rectangle)))