Yeah, like you said, those keys seem to conflict. I just tested a few combinations and the best I could come up with is using Alt/Option-# for ratings. So Alt-1 for 1 star, Alt-3 for 3 stars, etc. Alt-0 to clear the star rating field.
First you'll need the correct file. You can modify the Default file, but that's not a great idea. What you really want is the Resource.xml file in the Custom Resources folder. The full path should be:
/Applications/Media Center 20.app/Contents/Resources/Data/Custom Resources/Resource.xml
You can put the sample key mapping file there and modify it. Here are the lines I inserted at the end of mine to make it work with the Alt-# keys:
<!-- Ratings Commands, 0 to clear 1 through 5 to rate -->
<Entry Key="Alt;0" Command="10023" Param="0" />
<Entry Key="Alt;1" Command="10023" Param="1" />
<Entry Key="Alt;2" Command="10023" Param="2" />
<Entry Key="Alt;3" Command="10023" Param="3" />
<Entry Key="Alt;4" Command="10023" Param="4" />
<Entry Key="Alt;5" Command="10023" Param="5" />
If you get that working, you're most of the way there. The only problem is the Resource.xml file will get overwritten every time you upgrade Media Center! What you really want to do is make a Resources.xml file outside of the Application and link to it. I put mine here:
~/Library/Application\ Support/J\ River/Media\ Center\ 20/Data/Custom\ Resources/Resource.xml
To link to it, first go into the Application via Finder and locate the Custom Resources directory (inside the application). Delete the folder (assuming it's empty). Then you can create a link of that folder to the folder above where the "real" file is. Go to terminal and carefully paste in:
ln -s ~/Library/Application\ Support/J\ River/Media\ Center\ 20/Data/Custom\ Resources/ /Applications/Media\ Center\ 20.app/Contents/Resources/Data/Custom\ Resources
The Custom Resources folder should re-appear inside the Application in Finder as a shortcut. When you fire up Media Center, it should read it. Note that Media Center needs to be restarted to re-read the Resource.xml file.
I've left out some details here, but hopefully this will get you started in the right direction.
Good luck and let us know how it works out for you.
Brian.