Let's say I have a model, named MyModel, of which some instances have tags assigned. Is there a way, using the django-tagging manager classes, to get a list of MyModel instances with no tags assigned?

Or must I write my own custom queries against the django-tagging models?

It seems this would be possible to do in my own code if I could call TagManager._get_usage() and have it return a QuerySet rather than a list, because I could then apply my own filters to the returned QuerySet.