That's a nice ADC according to the data sheet, couldn't fine the paper via the link tho.
Recall Agilent used something like 80 individual precision uniform current sources for the 6 MSBs, then twiddled these to produce the desired result with DEM. The lower 8 bits were conventional R-2R ladder.
BTW think some of the newer 6 1/2 digit DMMs from the lower tier OEMs utilize a single chip ADC for the NPLC and higher speed digitizer functions as you've mentioned, altho KS and Keithley rely on a discrete ADC for the main ADC I believe.
Best,