Thanks for such a quick answer.
I think I may be actually confused about how Windows 7 file Explorer adds user-specified metadata (ratings, tags, description, etc.) to RAW files: I'd thought that Windows would use the RAW file WIC codec to add bits and pieces of metadata to the actual file in the formats WIC uses for such data, without reorganizing the RAW file data. So what does Windows use in this case?
For JPEGS, I'd thought that FPV (and Windows Explorer) would actually add some kind of XMP section to the file to carry the user specified, non-EXIF metadata.
I probably have to read more of the FPV information to sort this out...