Danny
Spotlight integration for OSX app created documents rely on the application developer building in support for Spotlight for bespoke file types.
The plugins mdimporter (metadata importer) for Spotlight and qlgenerator for QuickLook are included within the application package and registered when the application is first run.
Those plugins are application bundles in their own right. So without developer input from the LibreOffice developers, you won't get Spotlight integration.
Update 10/10/2012 According to the LibreOffice bug lists, SpotLight integration for LibreOffice file formats has been around for some time, however there have been some issues. I can confirm that 3.5.6 on Mountain Lion does now appear to work correctly.