(8 лет спустя) наткнулся на собственный вопрос, и пример будет (проверено в> = 1.11):
from django.db.models.signals import pre_save
from django.dispatch import receiver
@receiver(pre_save) # `post_save` also works
def callback_on_loaddata(sender, **kwargs):
# 'raw' indicates that loaddata cmd was issued
if kwargs.get('created', True) and kwargs.get('raw', False):
# mark on-going loaddata, call kwargs.get('instance').clean() etc.
...