A client company I work with is using Outlook 2007 and Cyrus-IMAP. Cyrus supports custom flags/tags/keywords and I've personally tested this functionality through both Thunderbird and Alpine--it works great. I've even checked the cyrus.header file on the server to verify that the flagging/tagging was actually occurring at the server level. It is.
So the question is, how can I do this through Outlook 2007? I'm willing to use add-ins/VBA if necessary. Client is pretty stuck on Outlook even though they won't use Exchange (they can't find a hosted Exchange plan they like).