(defun local/html-escape (str)
  "Convert STR to HTML escape sequences."
  (mapconcat (lambda (c) (format "&#%d;" c)) str))

(defun local/html-escape-region (beg end)
  "Convert region (BEG END) to HTML escape sequences."
  (interactive "r")
  (let ((str (buffer-substring beg end)))
    (delete-region beg end)
    (insert (local/html-escape str))))