My xrandr version is 1.1 so I should not reply...
it doesn't support any
On my laptop I have a ATI radeon card and when using an external monitor I'm able to select which monitor to turn off by means of the radeontool utility - maybe if you have such a card you can do the same thing.
And if you want to turn off both monitors, you can type: xset dpms force off
I know I haven't replied to your actual question, but if you didn't know about these tricks maybe this can help you