It seems, that it's not possible at the moment. Being able to see the ratings in those modes would be more important.
Which three viewer modes do you mean?
The standalone viewer, the full screen viewer, and the viewer pane ('Set Viewpane=toggle'). At the moment we only have the ratings viewed in the thumbnails, or in a column in the lister. Unless i'm not aware of an existing option.i
It's not built in, but there is a way:
I have tried it, but i can't get the focus in the background keeping up with the currently viewed image. I have tried adding Select Next to every button, but that doesn't work. Ideally the script would stay in sync with the selection in the lister, in case a new rating is considered.