>>> 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" ":-)")
>

Or, if you don't want to use the homoglyph face for such replacements:

(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 repl)))))

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