I have two thoughts on this:
1) if the files are DNG or JPG, then Capture One probably won’t see the XMP files because of the way Nitro names sidecars. You would need to rename them to match whatever they expect. This is not an issue for proprietary RAW (e.g. ARW, ORF, etc.)
2) There is no XMP standard for flags (location or values), so Capture One won’t read those either.