Is there any way to stop the Display Name being sent in mail?

I'm currently setting up a new Zimbra server (OS 4.5.4) to provide mail for staff and students in a school. We have 2 domains set in Zimbra, and, using a shared GAL. Due to child protection issues we cannot include students names in either their addresses or e-mails. For addresses we use randomly generated numbers, and this makes an easily searchable GAL (with accurate display name) vital.
The problem is that the Display Name is used in the FROM field (Eg: A Student <> ) of sent mail, which negates the obfuscated addresses. Is there any way to change this? Preferably on a per domain basis.

I've looked through the adminUI and searched the forum but havn't found an answer.