Thanks to both of you for your responses. All makes sense. Given I’m trying to mirror combinations of rating, flag and favourite, and most of the files in this instance are JPG, HEIC and DNG, I think the easiest way to deal with this is to simply use Nitro’s filters then export and import in batches, reapplying metadata on each batch as I import to C1. That will be a relatively quick process.
I realise now that C1 doesn’t have flags anyway, so even if the XMP standard did support them they wouldn’t come across.
I’ll also experiment with keywords in future.