The thing about the voltages is that you have such a wide range of desired currents.
Because of the wide current range, I think it makes better sense to have more than 1 voltage available to supply your current. The availability of several voltages at low supply impedance would allow a single set of resistors, switched betweeen supplies, to set current over the required range with more even precision.
The design approach I am taking here is to provide a set of 10 resistors switched in-and-out of circuit by mercury-wetted relay contacts. By selecting which resistors limit supply, current resolution of 1 part in 1024 can be obtained. Resolution can be increased (within limits) by increasing the number of switched resistors.
Just for fun, I have an inquiry with a supplier requesting 100-of prices for mercury wetted relays to switch 1mA at low voltages. Let's see what comes back!
whaddya say, daddles?