How to build a digital potentiometer with standard value resitors and MOSFETs?
If you wish to emulate a digitally variable resistance in a measuring instrument, viable scheme is to measure the voltage across the emulated device and draw a current corresponding to the desired behavior. This can be most easily accomplished with an MDAC (multiplying digital-to-analog converter). In fact, this is the normal approach used in RTD emulation calibrators. 

As to your approach, you could try using a series string of very low resistance MOSFETs with resistors in parallel, in binary sequence (1/2/4/8/16/32..) perhaps driving the gates with isolators. That would work with one polarity at DC (reverse polarity only slightly) and if you used very low Rds(on) MOSFETs you might be able to cover a fairly wide range of resistances. Keep in mind that MOSFETs have a bit of leakage and low Rds(on) types tend to have more, the leakage is very temperature sensitive and large MOSFETs have a lot of capacitance which makes them far from ideal, and even useless in some situations. The Rds(on) also has a (positive) tempco, but it’s closer to linear than exponential.