For someone with time, Photoshop savvy, and an important slide that suffers from the focus problem, I would like to recommend "A Multifocus Method for Controlling Depth of Field" at
http://www.sgi.com/grafica/depth/index.html The author, Paul Haeberli, takes two images of the same scene and performs some manipulations to create a single image that retains the sharpest portions of each individual image. When applied to scanners, the two images would be scans with the focus set to different places on the slide. It should not be hard to create Photoshop action that will combine the sharpest portion of two scans using this technique.