>> No, not any longer 😊.
>
> Can someone tell me an easy way to make that particular diamond 
> character display as the ASCII sequence `;-)'?
>

(defun set-character-display (char repl)
   (let ((c (char-from-name char)))
     (when (and c (stringp repl))
       (aset (or standard-display-table
 		(setq standard-display-table (make-display-table)))
 	    c
 	    (vconcat
 	     (mapcar
 	      (lambda (char) (make-glyph-code char 'homoglyph))
 	      repl))))))

(set-character-display "SMILING FACE WITH SMILING EYES" ":-)")