If you only want 1 per room, I'd find the specs for the LEDs (foward voltage, recommended safe current) and feed it with 12volts through a series resistor of suitable value. LEDs aren't rated by voltage, using a suitable resistor you could run them off 200 volts if you chose to (not recommended). Personally I'd wire 3 in series with 12 volts and a smaller resistor (it would use the same amount of electricity for 3* the light . Buying small (5mm) high brightness LEDs is cheap in comparison to the effort involved in positioning and wiring them. They can be had for pennies apiece on ebay. Running 1 LED from 12V is no more wasteful than using a 7805 to step down the voltage.
Carefully drill a 5mm hole (so as not to damage the surface) where you want the LED and stick a single LED in the hole from the back and hold in place with blutack/tape/hot melt/whatever and its barely noticable unless its on and dark in the room.