Troy blogged about the Technorati support for RSS tag feeds - it turns out that you can add support for that directly to BottomFeeder via the search building wizard. Under the Search menu, hit "Search Feed Builder". Then, click on the "Define Builder" button. You can fill in that dialog box like this:
Then, just go back to the search wizard itself, select Technorati from the drop down menu, and enter the tag you want to search for. Don't worry about spaces; the tool encodes that stuff for you.