It seems that we can catch the event MediaPlayPause
keyCode: 179
keyIdentifier: "MediaPlayPause"
This is a good news, but actually, I didn't manage to find a way to stop the initial event that do not depend of the browser, but the OS. Did someone find a way to cheat?
EDIT 04/2015:
I guess that's not possible to access the OS from a browser while doing JS.