Wow, that's sounds great. I believe we have enough room to put everything in place on one or even two levels if needed: e.g. ADC + MCU + Power on base PCB and DMM stuff as piggyback.
Would this still allow building a stand-alone unit in a minimal enclosure?
This would still allow a stand alone version with a minimal enclosure. The added part (otherwise provided by the EEZ box) would be the ground referenced part (e.g. ARM based µC + LDC module + some PC interface (USB ?)) and a crude power supply to give some 12 V (some 1- 1.5 W) - possibly from an external wall wart.
A point that may prove tricky is that in the EEZ box the boards are vertically mounted - not sure how happy the LM399 is with this.
From what I have found the space in the EEZ box es about 75 mm x 180 mm (maybe 200 mm). At least with my current density this would need 2 stacked boards. Likely split like:
A) ADC + reference(LM399) + ohms current source + some reference level generation
B) input protection, switching, main amplifier, current shunts and current switching.
Especially the relays (currently 5 x) may take up some space.
For the DC/DC converter and voltage regulation it is not sure which board would have space.
My current plan is to include some kind of internal calibration of the gain and shunt steps to get a artifact calibration like the 3458. For a DIY project it can be handy not to need many calibration points. This kind of needs current ranges down to µA and 1:10 steps. So there is no easy simplification with fewer shunts or only small current sources for Ohms. If at all one could simplify at the low current / high ohms (e.g. > 10 M) end. Also limiting the current to some 1-2 A can allow for more compact relays.