XGrabKey on the root window is how xbindkey does it. Be careful about having some alternative method of killing the grab though, it’s very annoying to have to go somewhere to ssh into your own box just to kill that process… And that’s why, if it was me, xbindkeys+”echo ‘moo’ > /tmp/moo-fifo” would be the way to do it. That way, you could also control it in any number of other ways you haven’t thought of yet.