Hello, while waiting for Linear's answer I give my thought.
Probably is useful to startup correctly the reference,
upon startup both opamp inputs are zero, so depending
on the unit's input offset, the out will try to go either up,
following starting the reference, or down locking it.
The diode will keep the opamp without feedback until
the out will not forward-bias the diode, so it will amplify
the noise on inputs with a gain of few million times.
So basically it's waiting that the noise will steer the out
the correct way (up) to start the reference.
Disclaimer: here is late, so I could have written strange
or plainly wrong things
So I asked also advice also on volt-nuts mailing list,
they are the ones on these sort of things.
Fabio.