What I have in mind is to connect a beagleboard to my laptop via ethernet, so the former plays audio and the latter is dealing with the visuals, communicating via OSC (mrpeach lib). Since my only controller will be an arduino nano, I only have 8 potentiometers and ten buttons, and I wouldn't like to spear any of these for something that can be done via the command line (plus, it could happen that I accidentally press the 'quit' button by mistake, during a performance). I tried your suggestions but they won't work. I'm on OS X 10.8.2 and via the Terminal I open Pd like this:
Applications/Pd-extended.app/Contents/Resources/bin/pd
pdsend is in the same directory, but I've no idea how to use it via command line to quit Pd..