Maybe try attaching the SP3 into a monitor, then turn the monitor off?So the SP3's screen can be shutoff when its connected to an external monitor. Maybe create some of windows script that will trick Windows into thinking that it is connected to another monitor and shut off the primary screen. Seems nutty that there isnt a way to shut off the darn screen. Every tablet I know of has this ability.
Even better if we could get a cable and short some channels to fool the SP3 into thinking it was connected to a monitor