I'm looking for recommendations for an XMP sidecar file editor. That is, something that will let me edit standalone .xmp sidecar files, not XMP data embedded within an image. Ideally Windows and preferably inexpensive (or even free). Also preferably something that doesn't impose it's own ideas about workflow - I'm really looking for "Notepad for XMP" rather than something like Adobe Bridge.
Use-case is that I am scanning and archiving old photographic media as DNG files. I'd like to create a standalone XMP file for each DNG to contain the metadata, and I'd like to use an editor that understands the XMP data schema.
(I appreciate that XML is "just" XML, and I can use a text editor to fill-in a boilerplate XML file, or a schema-directed XML editor. Those are my fallbacks, but ideally I'd just like a GUI editor.)