I've just tried to work on this one more time and finally had some luck. I wish someone could have given me more guidance.
All I wanted to was for <Space> to play/pause MC when I was focused on the window. It seems <Space> is mapped to other things depending on where in the window you are clicked. Overriding the key mapping in Resource.xml works but it has to be one of the last key mappings in the file. This now works no matter where in the window I'm focused. Yay!
I've also finally gotten a working solution for making these changes OUTSIDE of the contents of the Media Center application. Previously, we had to make changes toResource.xml inside the app, in it's file structure, and it would be over-written with each app update. I've taken an idea from Glynor and made a symbolic link from inside the app, to a directory in my Library. It looks like this:
/Applications/Media Center 20.app/Contents/Resources/Data/Custom Resources ->
/Users/bgentry/Library/Application Support/J River/Media Center 20/Data/Custom Resources/
The -> represents a symbolic link. Also known as an alias in Mac OSX. I had to delete the Custom Resources folder from inside the app first. Then create the symbolic link with the "ln -s" command from the Terminal.
Now I have Resource.xml in my Library folder as shown above and my custom keys work as expected!
Of course when I update Media Center, I'm going to have to recreate that symbolic link, but that's pretty easy.
Finally, I should mention that I also found out that the media keys on a Macbook Pro (and other Macs with these keys) control Media Center as long as it was the LAST MEDIA APP you were in. The keys I'm talking about are Play/Pause, track forward and track backward keys located just above 7,8, and 9 on the keyboard. These 3 keys control MC, even if you're focused on another app (like a web browser). This is fairly convenient, but the keys are small and I sorta prefer to just switch to MC and hit <space> if I want to pause.
I hope this helps someone else. It took me quite a bit of experimentation to get it to work.
Thanks,
Brian.