Yes, I realized that a little after I posted but I had to run off.
I suspect this points to a worthwhile refactoring of _get_queryset_methods to a function, say `get_exportable_methods`, that is defined next to the QuerySet class, and takes a QuerySet class and a filter. That would keep all handling of the queryset_only flags in django.db.models.query, which I'd find preferable.
But I wouldn't push for that too hard.