In a related discussion on the Arduino Forum, a writer comments that the ADC inputs have protective Schottky clamping diodes. This results in the following formula for current flowing into the A/D input:
I ≈ (9V - 0.4V - Vcc)/Resistor
Plugging in either the higher or lower values of voltage divider resistors in my examples works out pretty close to the values I'm seeing. With the Moteino in low power mode, the lower resistor of the divider is not grounded and the top resistor is connected to the battery supply, ≈ 4.6V, and to the ADC input through the diode (-0.4V) to the regulated Vcc, 3.3V. This results in about 1µA through the 1meg top resistor.
BTW, I'm pleased that the 3.3v regulator in the Moteino apparently draws very little quiescent current in the low power mode helping to achieve the 7µA I'm measuring.