Im not aware of any outlook ad-in that will accomplish this for you. Remember if each user has a profile on this local machine, each time the load there profile all data located on the server will open. This means their local .OST file can be accessed.

Here are some sites that have outlook add-ins:

http://www.addins4outlook.com/download.htm
http://www.xobni.com/lp/msftoutlookaddin
http://www.xobni.com/lp/msftoutlookaddin
http://www.office-addins.com/

hope this helps.