You can set star ratings in the viewer (without opening its metadata panel) using Star Ratings Buttons/Menu
But there isn't currently a way to see the current file's rating without turning on the metadata panel.
But you can see status icons (if the viewer's status bar is on and it isn't fullscreen), and buttons to set those would also work.
There's also the built-in Mark button, which lets you build up a list of flagged files for sorting them.
So there are a few options available already that might help.
I agree that it'd also be good to have a way to see and set ratings that doesn't use up much space, since it's so commonly done.