AFAIK all Greek letters will sort after 'z' - I use Ω because it sorts after all other greek letters. If I wanted something to sort after Ω I'd use 末 (Kanji - End) after that I'd give up and resort to numeric prefixes
I have no idea if any of this will work on your Kobo.
BTW a google search for 'what sorts after z' finds 34.5 million pages.
BR