You don't want the + and - to always change the volume. Only when in movie/music mode. We may have to do this through some introspection of some kind, assuming we can't completely reverse engineer those frappliances.
For example, BRMusicNowPlayingController and BRMusicQCNowPlayingLayer are both active only when the music player is on. You can view these using "heap", though that won't work for the volume control since there's a 3-second lag or so to use heap. Our volume control should only activate when these (and the movie equivalents) are loaded.
Adjusting the volume of the AppleTV with the remote's + and - button during playback.
A command line utility that does simple VolumeUp and VolumeDown seems the first step to see if it can be done. I have no programming experience with OS X, but this looks like what needs to be build into a command line tool: