In fact I've given it a go and it works OK. I use xrandr --fb to adjust the screen size. I've made this 3 pixels less wide than my actual screen resolution (1021x768) and the mouse pointer will now not go to the left most 3 pixels of my monitor. The missing pixels show up as black but metacity still maximises my applications to the monitor size (1024x768).
xrandr --fb 1021x768
It's not pretty and will not last beyond a reboot or adding an external monitor, but it doesn't seem to break anything.