ultra low noise reference and supply voltage with multiple ldo's in parallel
No offense, but I don't think you really want a 6 digit DVM. Suppose you're measuring a 1 V signal, and the meter displays  

$$ \large{1.02435 V} $$

The "5" are tens of microvolts. Are you really going to write down all those digits in you logbook? For everyday use I would write "1.02 V", for high precision maybe "1.024 V", but never more, but then I need a high precision setup in the first place. And the last digit won't be stable, even the "3" may switch to a "2" from time to time. What is it then? Also, measuring to better than 0.1 % (which is 3 digits) requires high precision for every component; the chain is only as strong as the weakest link. An 8 cm copper trace 0.2 mm wide on your PCB has a 0.1 &Omega; resistance, which will give an error of 0.1 % on a 100 &Omega; resistance. A 10 k&Omega; resistor has about 2 &micro;V Johnson noise over a 10 kHz bandwidth. On a hot summer day you may have different readings with the windows open or closed.  

Frankly, I would specify 4 significant digits; it will be hard enough to get the last digit stable and correct. And unless you work at FermiLab or CERN you're not really interested in more. It's not worth the trouble.

edit  


  " in my application I need this level of precision unfortunately"


Possible, yet extremely unlikely, even in a lab setup. Convince us. (By the way, do you mean precision or accuracy?) 



What's that success up to 5 digits? Did you really pay $17 for a single 0.01 % resistor with a 0.2 ppm/°C tempco? For 6 digits accuracy you'll have to build a temperature controlled oven for your complete circuit, or the 0.2 ppm/°C will render the last digit useless.

There's a huge difference between 6 digit resolution and 6 digit accuracy.