There was a java-based IPTC metadata panel tool for Bridge out there that would allow creation of custom metadata panels, but I dunno if it still works. Adobe has instructions for doing it here: Developer Resources
Otherwise, although a plugin like Search Replace Transfer can create custom fields within Lr, I don't think anything actually does custom metadata panels for inclusion in the file's XMP. But post if you find something.
Big Note will do that, but custom metadata can only stored in the LR catalog file and not in the file's XMP metadata:
Google Search: Lightroom XMP add custom fields
https://www.google.com/search?num=100&biw=1468&bih=985&q=Lightroom+XMP+add+custom+fields&o q=Lightroom+XMP+add+custom+fields&gs_l=serp.12..0i71l22.214.171.124.89126.96.36.199.0.0.0.0.0..0.0.... 0...1c..64.serp..0.0.0.95AZe2etmy4
I guess I don't understand how custom metadata can (1) be stored in the catalog file but (2) not be writeable to the XMP metadata, but that's probably my lack of understanding of how LR uses its database.
I ran your search above, but I had already reviewed some of the links in previous searches. No joy from this search.
I really liked a post from John Beardy in another thread which goes in part:
I've always thought it's a weakness of LR that it doesn't allow users to create their own custom fields and instead forces people to add non-standard metadata to keywords. While keywords have the flexibility to record all sorts of data, and are XMP-friendly, there's not the kind of strong control that custom fields provide (eg a "to do" smart collection that checks custom field A is empty and custom field B is empty).
My interest is the same as that of people who work with large studios: The standard metadata is inadequate to describe fully the image. In my case, it's a hobby in which the metadata for an image is very important, since many images are in effect a historical record.
You should definitely check out the instructions for custom metadata panels I referred to (or try these; think they still work: Metadata Read/Write Mapping - ResourceSpace Documentation Wiki). That's what the XMP spec is all about. Generally only in-house or enterprise users might wanna do that, and there obviously doesn't seem to be a consumer-level demand or there would at least be plugins.
But consider that getting such a panel is only half the battle. To make it really useful, other applications would need to be able to access it. As of now, I have trouble with even some standard IPTC extension stuff like locations shown and created. But the info panel would be available in Bridge. Lr, AFAIK, doesn't support them, at least directly. How Lr uses these is unknown to me.