Currently, only [extension by grum] Advanced Metadata handles XMP metadata (but this plugin doesn't fill the photo properties yet, they are stored in a distinct data model)
I was going to post a separate request, but I guess this post is similar enough. I love the exif extension (Advanced Metadata plugin) and the tag cloud is very nice too. I would love to see both of these leveraged by directly importing exif data as tags. This would allow filtering based on so many parameters such as lens used, camera used, etc., without the user having to re-create these tags in Piwigo.