I have Excel 2000 and the save preview picture option is there and works. You have to turn it on individually for each file you save, via File -> Properties in Excel.
(You could probably make it the default for new documents by editing the template document that Excel uses when creating new documents. Existing documents would still need to be edited individually.)
Once an XLS file is saved wht the option turned on, Opus will then show thumbnails of it once you close the document. You won't see thumbnails for Excel documents that are currently open in Excel because of the way it edits them (you'll see icons for them instead).
(You've also need to turn on Preferences - Lister Display Modes - Thumbnails Mode - Use shell image extraction for formats Opus does not understand within Opus, but you must have already done that as you're getting Word thumbnails.)