No, you are right, this should be part of the standard install and it isn't. I don't think it was even possible with Eden really, but now with JSON it is.
Here is the command:
<?xml version="1.0" encoding="utf-16"?>
<command id="399" name="set rating {1}" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>Player.GetItem&&"playerid":0</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.ParseTokens</cmdType>
<cmdString>{item.id}</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.ShowText</cmdType>
<cmdString>{LastResult}</cmdString>
<cmdRepeat>0</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>AudioLibrary.SetSongDetails&&"songid":{LastResult}, "rating":{1}</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>set</phrase>
<phrase optional="true">song, track</phrase>
<phrase>rating</phrase>
<phrase optional="true">to</phrase>
<payloadRange>1,5</payloadRange>
<phrase optional="true">stars</phrase>
</command>
Note that XBMC seems to have a small bug. The fullscreen viewer will not show the updated rating until you play another song and then come back to the song you rated. Then you will see the correct number of stars.
However, if you are in "Library Mode" and using the "Media Info" view, you will see it update immediately.