I wrote a simple plugin for site wide tags. Did it in 3 hours, of which 2,5 were research, so don't expect heavy features. The code is just under 100 lines. For the coders it should be easy to adjust to specific needs.
It works with a multidomain install, thats what I use it for.
The plugin uses the tables used Simple Tagging plugin by Amaury Balmer. So you should really use that plugin too. With a little adjustment UTW could be used. Although I don't trust UTW because of the comment bug.
Have fun :)