Hi,
There is a partial solution to your issue. Unfortunately there is no option for telling the document selector which documents are available to select, but you may set Starting alias path option for a user, which shows just chosen branch of the Content tree and therefore it is shown the same way in the Document selector form control.
Also I believe that it should be possible for you to "hide" certain documents with permissions -> If user does not have read permissions to a document it should also not be visible in the selector.For this option please see
http://devnet.kentico.com/docs/6_0/devguide/index.html?hiding_documents_in_the_content_tree.htmKind regards,
Richard Sustek