Okay, from your response to my comment I think I can be more help with a full answer.
Build your simple Resistive divider. This has many advantages, for one size, another is cost.
Now, you are going to use a microcontroller also, this is where you need to characterize your error. use a voltage generator that is very accurate, now check voltages and see what error the uC actually measures.
Now is when it gets fun. There are a number of things you could measure.
Accuracy and Precision
The biggest thing to measure here is if the error at the points is repeatable. IF every time you sweep you get a different error, or if your sweep following a different path gives a different error at the same point, this is not an option. This often happens with leakage currents. The important part here is that you do not need high accuracy, just a high degree of precision. If you can characterize your error, your uC can correct for it. If you have a large variance you need to change your solution.
What is an X point calibration?
Now, if you have a high precision, as discussed before, you can move on to correcting the accuracy. Now, if when you graph your input voltage versus your output voltage you are going to have to decide the number of "reference points" you need. Nice devices allow a 1 point(or zero point, no need for calibration). Many temperature probes.
One point calibration
Both lines have the same slope, but an offset, so you just need to find the value you must add to a data-point to correct it. This is an ideal situation, as any new calibration only needs a single data-point to calibrate again.
Two Point calibration
both linear curves, possibly an offset and there is a slop difference, you only need two reference points and linear interpolation to extract your offset. This is still relatively easy, you just take any point, multiply it by a scalar and add an offset.
As you can see, it gets messier the more points you need. At some point it is easier to just take every datapoint and correlate it to the actual value. For example, finding that 0000 is 1V, 0001 is 2V, 0002 is 1.5V. This is messy, and this still only works if the offsets are repeatable. This can happen though.
Summing it up
I hope this helps, tell me if you need more clarity. If you end up with a varying leakage current that is not dependable, it is time to just deal with needing a buffer, or something of the like.
I can see a possibility of high error from leakage, but I would bet that the error is relatively small over most of the range, and when it is there can be easily corrected.