Found a solution using overwirte of response_add and response_change from ModelAdmin:

class AuthorAdmin(admin.ModelAdmin):
    inlines = [BookInline]

    def after_saving_model_and_related_inlines(self, obj):
        if obj.create_report:
            obj.write_report(name=obj.name, books=obj.book_set.all())

    def response_add(self, request, new_object):
        obj = self.after_saving_model_and_related_inlines(new_object)
        return super(FooAdmin, self).response_add(request, obj)

    def response_change(self, request, obj):
        obj = self.after_saving_model_and_related_inlines(obj)
        return super(FooAdmin, self).response_change(request, obj)

Source: https://igorsobreira.com/2011/02/12/change-object-after-saving-all-inlines-in-django-admin.html