No need to smooth the connection between a battery and a linear regulator for the sake of measuring the level. Even with a switching regulator as I'm using it's not necessary.
Just limit the current that can flow into the pin to a low value - analog inputs are high impedance (10^10 ohms or more), until the protection diodes conduct. Limit the current and in the inbuilt protection diodes do the rest. A few kohms will be enough.