In the PIC12F1822 and PIC16F1823 there is not a DACNSS bit. The drawing (Figure 17-1) and references to this are incorrect and the register bits are correct as published. The negative reference is always connected to Vss.
The PIC12F1840 DACCON0 is incorrect and bit ‘0’ should be unimplemented.
There is no negative Vref pin on the chips so that should've been the clincher for me.