While reading the Solr-Search-Docs I stumbled about the exclusion-chapter ( Solr Search Application (XWiki.org)). Is it also possible just to exclude the “secret” areas?
Yes, you can configure exclusions in the search administration. But this will be for all users. If you don’t mind touching the code (possibly leading to conflicts during upgrades), you could also add the excludes in a filterQuery in Main.SolrSearchConfig if the current user doesn’t have access to them.