Create small variations in voltage for opamp reference pin
However i'm aware that tiny differences in the bridges, opamps and
  resistors will require slightly different offsets for every bridge
  (i'm expecting something like +-0.05V would do)


I wouldn't bother trying to do this in hardware; I'd just compensate the ADC reading digitally to overcome the slight discrepancies in offsets. I'd produce one solid 1.65 volt reference voltage and feed this to all AD8422 InAmps.

After all, if there are resistor variations that produce voltage offsets, those variations will also produce gain variations that are likely to need compensating in software.