It would simplify the circuit if we can place a potential divider of say 10kR
10kR to ground before the +ve input of the amplifier. The data sheet states the
reference must be buffered. Would the potential divider be too large a load, it
add 0.12mA to the internal 0.5mA internal current sink.
The bottom line is the on-chip reference is designed to drive the ADC and
offset only. The output drive of the Vout pin is not tested and not guaranteed
and should be buffered if used elsewhere in a system.
A 20k resistor to ground will not add significant load to the reference output
and is likely to produce only a small gain error. But the lack of an explicit
product test puts your design in “no man’s land”.
A dual opamp, one to buffer the reference and another to buffer the output of
the voltage divider is the most reliable design.