I don't think that PWM is a suited methode to apply ultra high precision reference voltages. I would use precision voltage generators in combination with high reliability relays (inert atmosphere arround massive gold contacts).

When using relays in precise analog, remember to use 2 contact sets in reverse polarity, the contacts give a small EMF.

like this

 
|> |  | <|
|  |  |  |
|  |__|  |
|        |
0        0

Erik