Today, thanks to this Mac OS X Hint, I have an answer. Go to FEEDcombine, click on the link to Make Feed, and then fill the list with up to 10 feeds and click on go to feed. Add the feed to Safari and then select the feed in your RSS Visualizer preferences in the ScreenSaver tab of the Desktop / ScreenSaver control panel.
Unfortunately, the RSS Visualizer caps its display at 10 stories, so you should do no more than 10 feeds at 1 story per feed, but you could do 2 feeds at 5 stories per feed, or 3 feeds at 4, 3, and 3, etc. If you need help with the math, Laurie, feel free to ask. ;-)