Harvest energy from ultra low current and high voltage source?
There is almost no power there- 50nA at 10V is 0.5uW. 

Yes, you could probably find a way to harvest that 0.5uW (for example, charge a capacitor and then kick start a converter intermittently and store it in a super cap or battery) it's hardly going to be worthwhile in most cases. By comparison a 2032 Lithium button cell has 235mAh capacity which is probably around 10 years supplying 0.5uW including self-discharge. And you're not going to get 100% efficiency harvesting that 500nW, as you've observed it's hard to get more than 0%. 

A Darlington would amplify the signal but it does not create energy out of the ether- it requires another power source.