OS X :: How To Sleep Only The Primary Display

I use my Macbook + Boxee to watch videos via a projector as my secondary display, but when I'm watching videos the bright macbook screen can be distracting next to my projected display. I'd naturally 'sleep' my display (I even have that function set as one of my 'hot corners') but if I do this then the secondary display, i.e. the projector, also loses picture. I'm therefore left with the options of dimming the brightness of the macbook's screen right down to its lowest setting (which seems to be 'off') or shutting the macbook lid, which naturally causes the whole system to sleep unless I use one of those reportedly dodgy kernel-hacking apps like InsomniaX.