can I -- just before generating the slideshow -- simply "select all" of the versions I want in the slideshow, raise the quality on the previews in "Preferences" and then "Regenerate Previews" -- and then generate the slideshow.
Yes you can do this.
You will either need to delete the previews for the images you have selected first or else force the previews to be re-generated.
On the second page in the Slideshow chapter of the User Manual, this section is separated and highlighted:
"About Displaying Sharp Images in Slideshows
To display slideshows at the best quality, you should set Aperture to create high-resolution previews. In addition to setting previews to their highest resolution, you should also use full-size previews to prevent upscaling with large displays. If previews are not available, images in the slideshow may not be as sharp as they would be otherwise. For more information about creating high-resolution previews, see Working with Preview Images." (http://documentation.apple.com/en/aperture/usermanual/#chapter=22%26section=2)
The Previews page (linked in the original) is one of the most comprehensive of all the help pages.
(Sent from my magic glass.)