I just happened to stumble onto an OmniFocus article from the Simplicity Is Bliss blog. The author talks about how he uses a custom search with saved perspectives to create his own tagging system.
So this might be another way of using the notes section for each task/project as a way to create a "tagging" system.
Enjoy...
http://www.1klb.com/blog/2012/4/15/h...omnifocus.html