How exactly do you suppose this to function?
This is what I put into a online simulator. If I change the impedance on the right then I can adjust the trim pot and it changes the working range.
This is a digital input, not an analog one. Re-read what I wrote about input protection. How much current flows into the input? Draw a schematic including the input protection as in the article you are referring to (hint: Figure 6).
Do you mean what you wrote when you said to use an input series resistor. Is the circuit above what you meant?
EDIT: I have since found out that the impedance will be minimal. It is just the resistance of a short cable. So less than 1 ohm. Also this device will go into deep sleep in which case there will still be the analog but no power to the FiPy. Won't this then apply a voltage to VDD?