Ivan,
I don't know if you have AdobeLibrary3.jsx or not. You can get it by downloading the Import from Camera script on Adobe Exchange.
In it, I wrote an object to help with things like namespaces.
Here's an excerpt of that code that defined "namespace objects" that are used to get/set metadata
new MdNs.MetadataNamespace( "IPTC Core", "http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/", "Iptc4xmpCore" );
new MdNs.MetadataNamespace( "EXIF", "http://ns.adobe.com/exif/1.0/", "exif");
new MdNs.MetadataNamespace( "Dublin Core", "http://purl.org/dc/elements/1.1/", "dc" );
new MdNs.MetadataNamespace( "TIFF", "http://ns.adobe.com/tiff/1.0/", "tiff" );
new MdNs.MetadataNamespace( "XMP Basic", "http://ns.adobe.com/xap/1.0/", "xmp" );
new MdNs.MetadataNamespace( "XMP Media Management", "http://ns.adobe.com/xap/1.0/mm/", "xmpMM" );
new MdNs.MetadataNamespace( "XMP Rights Management", "http://ns.adobe.com/xap/1.0/rights/", "xmpRights" );
new MdNs.MetadataNamespace( "XMP Basic Job Ticket", "http://ns.adobe.com/xap/1.0/bj/", "xmpBJ" );
new MdNs.MetadataNamespace( "XMP Paged-Text", "http://ns.adobe.com/xap/1.0/t/pg/", "xmpTPg" );
new MdNs.MetadataNamespace( "Photoshop", "http://ns.adobe.com/photoshop/1.0/", "photoshop" );
// new MdNs.MetadataNamespace( "Stock Photo", "http://ns.adobe.com/StockPhoto/1.0/", "bmsp" );
new MdNs.MetadataNamespace( "Adobe PDF", "http://ns.adobe.com/pdf/1.3/", "pdf" );
new MdNs.MetadataNamespace( "Camera Raw Settings", "http://ns.adobe.com/camera-raw-settings/1.0/", "crs" );
// new MdNs.MetadataNamespace( "PNG", "http://ns.adobe.com/png/1.0/", "png" );
This snippet shows all of the standard XMP namespaces that I was able to find.
Also look at the MdManager class for getting and setting metadata for a given thumbnail
Bob
Adobe Workflow Scripting