My suggestion would be to setup a slipt domain:
http://wiki.zimbra.com/index.php?title=Split_Domain then migrate the users a few at a time instead of syncing twice a day. You may have to do some after hours work when the user or users have left for the day, but that is the life of an admin. If for some crazy reason you decide not to implement Zimbra you can always migrate the user back. IMHO I believe you are making it out to be much more difficult than it really needs.