I would assume a current source directly interfaced to a 0,1R resistor would introduce error voltages trough the silicon components in the source? Theoretically it should work, but at this little voltage levels and the corresponding measurement times i rather think about everything twice, before trusting any measurement.
Apart from that i dont have a good current source at hand (need to get my hands on a Keithley 220 or similar in the future) to realize the suggestion, but i have the resistors lying around.
Yes its a DC-0,2Hz bandwidth amplifier. Biggest enemy in this case, after getting noise bandwidth sufficiently low, is thermal drift.
I dont have the AoE-book yet, but i plan to get it.