I have a form in my Django app (not in admin) that allows staff members to select a user from a dropdown.
forms.ModelChoiceField(queryset = User.objects.filt
You can override the field with a custom ModelChoiceField and change the label_from_instance function to return get_full_name instead. See the docs for ModelChoiceField: http://docs.djangoproject.com/en/1.2/ref/forms/fields/#modelchoicefield
ModelChoiceField
label_from_instance
get_full_name