If your battery voltage is less than one diode-drop above your CPU operating voltage, switching off the divider using an I/O pin set to "input" will work. If the battery voltage is higher than that, the difference will appear across your divider, just as you say, with the associated current draw.
If you use a large value resistor like 470k it's not going to be too much battery drain, maybe a few microamps ? With that high an imput impedance, you might also need to add a capacitor from the ADC to ground to make the ADC reading valid, though.
All this is just theory- I haven't tried it.