Doesn't seem to make a difference if it was added to the world or not. There is no problem of course if the parent Object3D is not a dummy, as you probably already know from your recent tests on the billboarded child scale-inheritance issue.
--EDIT--
Using a "new Object3D( 0 )" for the parent has the same problem. Adding it to the world and/or calling "build()" on it has no effect either. However, making it a "new Object3D( 1 )" and calling "build()" seems to work (whether adding it to the world or not). In either case, the problem only occurs if the child Object3D in question is billboarded. Also, it doesn't seem to make any difference how far down the chain of inheritance the billboarded object is (it could be the great grandchild of a dummy and it will still be invisible).