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.