gistfile1.py

class Account(models.Model):
    pass

class CustomUser(AbstractBaseUser, PermissionsMixin):
    accounts = models.ManyToManyField(Account,
                                      through='CustomUserAccount')

    changed_by = models.ForeignKey(CustomUser, null=True, blank=True)

    @property
    def _history_user(self):
        return self.changed_by

    @_history_user.setter
    def _history_user(self, value):
        self.changed_by = value

    ....

class CustomUserAccount(models.Model):
    user = models.ForeignKey(CustomUser)
    account = models.ForeignKey('account.Account')
    role = models.CharField(max_length=20)

    changed_by = models.ForeignKey(CustomUser, null=True, blank=True, 
                                   related_name='changed_by_user')

    @property
    def _history_user(self):
        return self.changed_by

    @_history_user.setter
    def _history_user(self, value):
        self.changed_by = value

    ....

register(CustomUser)
register(CustomUserAccount)