I searched around for you a bit today, and could only find some vague
references to "improving" appetite with B12 (when one is run down and
deficient).

You know, historically B12 was used commonly in the "old" days when
patients complained of feeling fatigued. Doctors routinely gave B12 shots,
and I recall getting some in the '70's. Back then not much was known
medically about many things so doctors did what they thought might work.
This resulted in a controversy, that giving B12 willy-nilly was quack medicine.
This backlash persists to this day and the overreaction to ignore B12 completely is just as bad or worse.

So I think you should discuss this with your doctor. Why he thinks you need
such high doses. And if he gives you some good reasons, with some studies/references perhaps, you could share them here with us?