Volume control
Volume control
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:
Apple Developer imsoundmgr documentation
and/or
Cocoadev.