Thanks,
R0J0hound! I'll use your advice:
So it seems a good solution would be to create a layout, put one instance of every object, and move that layout to the top of the layout list.
It still sucks that there's no easy way to find the current default instance. In a big project I often waste a lot of time looking for it.
Another similar problem is finding the layout where a global layer is defined. Why do I have to open every single layout and check if the layer is overridden there or not?