You can only use the document library name in the <item> element in the configuration file:
This is a limitation of the current implementation of our File browser content provider. As you already know, we can not access libraries from sibling sites - only the current site and its parent sites. We are working on fixing this in a future RadEditor for MOSS release (i.e. you will be able to use the full path to the document library in the <item>).
The only workaround I can offer you at the moment is to use the MOSS Link Manager to insert links to documents. Unfortunately, the MOSS Link manager only works in IE and Safari (does not work in Firefox).
the Telerik team