, the feature, as you describing it, is not implemented in Outlook.
Probably solution with attachment is the only one,
but you can make it a little more developed.
Instead using Word file you can use Excel or Access file (export all contacts first to it) and setup it the way that it would be pulling data from Outlook contacts and constantly updating itself (excel or access file). And in that database you can create separate columns to store your private data. Make it password protect and have it attached to the contact.
As another non-related measure you can activate screen saver and make it show logon screen with your password protected Windows account and non-password guest account when on resume. So if someone decides to play around with your computer, he will only be able to get into Windows as a guest user and would not see your data.