The Keysight 34461A arrived today! I completed the current reference board yesterday...almost perfect timing. I plugged in the 34461A and let it warm up for a couple of hours at room temp (72°F/22.2°C). I adjusted the current reference potentiometer to put the three current reading errors approximately centered between the three readings. I took these photos of the front panel for each of the three  current reference values, with Aperture set to 100 60Hz line cycles, and Auto Zero on.

I received a calibration certificate with the unit. Here's the relevant data:

     DCI 10mA on 10mA range. Reading: 0.0100000 Error: -0.0003%. 

I don't know what these guys are smoking, but the above line of text doesn't make any sense. I believe that if they are truly reading 10mA on the 10mA range then the reading should be 9.99997. It seems that they have the DMM range set to 1A, which would yield the correct reading of 0.01 for a 10mA current. They did not calibrate for lower values than 10mA! Let's assume that the 34461A is perfectly accurate and go from there. 

The first reading on the 100µA range has an error of +0.0018% (90 day spec = ±0.065%), that's 1.8nA. The second reading on the 1mA range has an error of -0.0028% (90 day spec = 0.036%. And the third reading on the 10mA range has an error of -0.0052% (90 day spec = 0.032%).

So it appears that the current reference accuracy is at least 6x better than the tolerance of the 34461A. Not bad!

[Edit 2019-05-21]:

I found a major flaw in the layout when I proceeded to take a 10mA measurement with my old HP972A DMM. The ground lead of the 1000µF filter capacitor is dangerously close to the common probe pin. The probe slipped off the lead and shorted the meter across the 15V supply. That blew the protection fuse and killed the BAV70 diode clamp on the 10mA output. That was the first time that I'd ever blow a fuse on a meter...since the 1960's!

I desoldered the filter cap and installed insulating sleeves over both leads before reattaching it. 

The other problem that I noticed is that the HP972A measures the 1mA source inaccurately -- about 20% low on the 4000µA range, but gets the correct value (0nly two digits: 0.99mA) on the 40mA range. It turns out that the meter uses a 550Ω shunt resistor to measure the current on the 400µA and 4000µA ranges, and an 8Ω resistor to measure current on the 40mA and 400mA ranges. The 550Ω resistor develops 0.55V across it when measuring 1mA -- this is enough to turn on the BAV70 diode and siphon off some current to the supply. The solution is to put a small battery in series with the DMM to boost the voltage above the supply. A 1.5V cell should do the trick. 

If I was going to redesign the circuit I would add another BAV70 diode between the anodes of the three clamp diodes and the positive supply rail. That would provide a compliance of nearly 1V.