pjb

(defun append-char (start end string)
  "Append character(s) at the end of region"
  (interactive "r\nsChar to insert: ")
  (save-excursion
    (goto-char start)
    (while (and (<= (point) end) (re-search-forward "$" (1+ end) nil))
      (insert string)
      (forward-char 1))))