How do you configure ADC7 on the Arduino Pro Mini as a digital output?
This may be a little left field, but I use the ADC6 and ADC7 inputs to detect switches.  I just don't use the digitalRead() function, but instead use analogRead() function.  Using a resistor network and switches and some decoder software code in our sketch, we can use a single input pin to interface with many more switches.  The switches can only be on one at a time, but in many instances, this would be OK.