I'm having *exactly* the same issue and I think it's because there is a distinction between "MPlayer" and "MPlayer OSX".

The first is a command-line application. The second is a GUI app that wraps up the command line app.

So when you run it from the dock you're running the GUI app, and Remote Buddy works fine. When we run it from within MythTv we're using the command line application and Remote Buddy doesn't know to do anything.

I can see two ways to work around the issue:

1) We may be able to use the GUI app from the command line. 
2) We may be able to configure Remote Buddy to know what to do with 'regular' MPlayer.

I'll look into it a little more and keep you posted...