The directory display can be displayed only by first name, last name format or last name first name format. To sort this you may have to point the directory URL to an external app (third party or custom developed) which can sort the display the way you want and then output that in XML format.
Another alternative is to add bogus alphabet prefixes (A,B,C etc) in front of the existing first name or last name (depending on how your directory format is) and that way you can control which names can show up at the top of the list.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus