Would it be possible to spread out her 900 calories through out the day? Instead of 3 square meals a day and a snack, just let her eat the same amount of food all day. It would be extra work for you, but it might nurture her need to continually be eating. Let her have a continuous stream of food. Let her eat Cheerios all the time.
Did you learn anything useful about your dd from the psychologist? Is this typical behavior for these orphans?
Perhaps I'm not getting the full scope of the issue. While her eating habits and behavior don't seem typical, and they do seem extreme, I don't exactly see anything wrong with it, other than it's totally stressing you out. Understandably!