Robert,
XMP Custom Panels are part of the IPTC Core spec.
While I agree that there is a document titled:
"IPTC Core" Schema for XMP
Version 1.0
Supplemental Documentation
Custom Panels User Guide
And that it shows the XMP panels being used in Photoshop CS, nowhere that I can find does it document the layout of the custom panel files.
We support every field described in the IPTC4XMP specification. While I think it would be nice to support custom XMP panels it is not necessary (or even desired) to support the same IPTC Core panels that Photoshop uses to allow editing of metadata because it provides a poor user experience (several fields appear in more than one location, like how Author in the Description page is later called Creator in the IPTC Contact page which is both confusing and redundant.)
I don't believe that the IPTC Core Schema for XMP specification document says that you must support XMP custom panels. It just specifies the schema of the XML, names fields and explains their usage. I just read through it again, and nowhere does it say anything about XMP panels.
If I'm wrong about any of this, please do correct me.
Thanks,
-Kirk