The closest thing would be to mirror, and turn the brightness of the internal display to zero. The LCD pixels may still be getting power, and the GPU may still be doing extra work (I know my old PBG3 was slower when mirrored--may no longer be true) but the backlight will be off.
I do that sometimes when watching movies on TV--it's distracting to have them also be on my PowerBook screen at the same time.