The number of tags the widget can follow is very limited (because of the Twitter API) so it does not accept arbitrary tags.
I would have suggested using the Identica widget (which does accept arbitrary tags), but Identica is closing down at the end of the month, making way for the new Pump.io service. I haven't made the time to create a Pump.io widget yet.
So I don't have a good solution at the moment.
Thanks Jim. Micro-blogging while very useful seems to have undergone lots of changes of late. Maybe as Facebook/Google start to make inroads. Unfortunately twitter recently made some filter changes to their API for hash tags so when I used it in class only "selected" tweets came through. Actually made using twitter nearly useless to use as a teaching tool (in the end none of my students were posting tweets!).