Hello JP,
well, you already indicated the best option: googleon / googleoff.
http://code.google.com/apis/searchappliance/documentation/60/admin_crawl/Preparing.html#pagepart
indicates that the 'snippet' googleon/off would be the option you are
looking for.
If this does not achieve the results for you, you can fill the
description meta tag in the page with desired snippet texts. This will
automatically be used if either no content is found to generate a
snippet (i.e. no "q" parameter value provided for the search, all
content excluded for snippet). But you can also rewrite your frontend
xslt code to use the meta tag instead of the generated <s> tag.
Cheers
Mike