I think rating the current song is great, but rating of previous songs is not really necessary because it can be achieve in the main interface.

The GNOME Music Applet allows this:

http://web.ics.purdue.edu/~kuliniew/music-applet/

and it's unlikely to added to the rhythmbox notification icon as the HIG advises against overloading systray icons with functions (they are supposed to be for notifications primarily, not control, AFAIK).

IMHO it's not overloading to put five stars in the notification that tells you which song is being played to rate the previous song

I have said previous song mainly because if you are listening a new album you don't know by advance if it's great the next song rather the previous one

maybe a good solution is move from the notification bubble to notification applet

i.e. now if you right-click you have:
play
previous
next
show player
display notifications

so adding the current rate (the starts) and allowing to change it would be nice :)

there is no status icon any more