Thank you for your inputs. To answer some questions: I want to use 30M voltage divider because it drains ~100nA ~ 1mAh/year. 200mAh coin should last 200 years which seems good compared to its shelf life — and my life expectancy too. Saving complexity of a transistor powered voltage divider looks like a good deal. I think I will try it and see if it will give readings of reasonable accuracy.
Grumpy_Mike:
Is rather silly, when a fingerprint on a resistor can easily measure 10M.
Can you be more specific about this? How you make such fingerprint and how you measure the resistance to get 10M? At high voltage/frequency maybe? I used my cheap DMM to measure resistance of my 10M resistor (fingerprints already included) and got readings between 10.05 and 10.07 MOhm (the DMM is cheap, it takes long to settle). I tried touch and rub the resistor by bare fingers but the reading didn't change. I even used a hand creme to make my fingers "more printing" but still nothing. Another of my 10M resistors read 10.15 +/- 0.01 MOhm so I concluded my fingerprints has resistance many GOhm...