After some digging I figured out how to do this. In McAfee email gateway 7.0.1 you have to go to Email settings and then to DLP and Compliance. Then go to Compliance and Dictionaries and Add a new Dictionary. For details add the string you wanr to use like #secure# and specify the dictionary to look in the subject line.
After you create the dictonary you go Email policies and create an Outbound policy and in the Compliance section under rules reference the dictionary you created. I have tested this many times and it work. In an Outlook email you can put the #secure# in the subject and the Email gateway will scan this and instead of sending the email though it will invoke the secure web client. Works good.
McAfee should put this in their documentation, I found no references to this anywhere and even the McAfee support didn't know about it. Just trial and error discovered it.