An image viewer that
Can display (the embedded JPG inside) RAW files.
Supports rating images and can write the ratings into the files.
Does not need to "import" the images.
I don't know any. Shotwell requires images to be imported and Geeqie cannot write the ratings into the file (as far as I can tell). Gthumb cannot display the preview JPG in a RAW file.
Geeqie can handle RAW files correctly. If you want to see thumbnails in nautilus you can use ufraw and create a thumb-nailer. It uses the raw embedded image and is the faster thumb-nailer I've seen. To do this, first install ufraw-batch. Then create a text file called /usr/share/thumbnailers/raw.thumbnailer and copy this:
Probably is not exactly what you are looking for, but I use Geeqie and use the keywords to rate them (you can add new keywords if needed). To select the images you right-clik on the keyword and connect the keyword to a mark.
And you can also batch copy or move the files. Select the images, right-click on the keyword and connect to a mark, the select the mark and on the icon panel now you only see the selected images. Then you can click on an icon and then Ctrl-A to select all images, righ-click and press copy.
Geeqie doesn't need to import images.