Okay, now we are getting somewhere.

By itself, .instance_of(*models) will actually call select_subclasses(*models),
which results in just those objects being cast to subclasses.

However, if you want the casting to grandchildren (which is supported only in
django 1.6+), then you may use an extra call to .select_subclasses() (without
arguments, to select all subclasses).

It doesn't deal with @kezabelle's discussion of mixed-use of select_subclasses/
instance_of, I still need to look at that.

But it does pass all current tests...