I have part of an answer. To link directly into the Mac App Store you need to use the MAS protocol which is “macappstore:”. This can be found by looking in the info.plist for the MAS app.
Some experimentation has found that using part of the URL from the link to an app will work in the MAS app. So if I copy the link to my app from the MAS app it looks like this:
http://itunes.apple.com/us/app/ringer-ringtone-maker/id402437824?mt=12
Of course using this does not open directly in the MAS app. But you can remove the store country designator and the name of the app and add the MAS protocol and you get this:
macappstore://itunes.apple.com/app/id402437824?mt=12
Which opens the main page for an app directly in the MAS app. I have not yet found a way to link directly to the rating page. Since the rating section in the MAS is just a part of the main page that is revealed it is possible that there is no link directly to it. I would love it if that were not true.
Perhaps someone else can find this last bit. In the meantime I plan on using the link to the main page as a fallback until the rating page URL can be found.