Finally! Thanks a million, Andrew! After rooting through a twisty little maze of online suggestions only to be stymied by the inability of the Carbon version of Emacs to display certain glyphs, and nearly giving up, I see that Andrew Choi has released the notes on building a Carbon XEmacs. Well done. And it handles UTF-8 files beautifully:

Note to self and others: don't try to follow his build instructions inside an Emacs shell buffer - it will fail.

Here's a binary (49M) with xemacs-sumo-package and xemacs-mule-sumo-package installed and interned inside the application bundle. It's had both of Andrew's patches applied as well. All built according to Andrew's notes.