Is there a possibility to get only the items from documentlibrarys where the documentlibrary is searchable. I think the property on the splist is nocrawl.
I found a link where is written that there is an attribut "WithIndex" but it slows down the performance. http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spsitedataquery.lists.aspx
Has anyone an idea?