Not sure if you found the way, but I was needing to do the same as you, and discovered an answer:
I created a custom namespace by editing the manifest and properties xml, that comes in the Generic Panel folder, that comes with the XMP File Info SDK, changing custom to your namespace.
Then in the createContainerItem you use the same custom namespace - and it worked. You should be able to create your Flex Patchpanel, for custom XMP user interface, in a pallete, and / or as a custom panel inside of the File Info dialog.
unfortunatly i dont think that is what i am looking for.
My major need is to do this in a jsx indesign server script.
I think it is crazy for adobe to charge what they charge for indesign server and not support basic custom xmp injection.
What i want should be simple.
I want to register a namespace at.
http://adsml.org/xmlns/ using adsml-at:
and i want to create a key value pair for something like
adsml-at:BuyerName = John Doe