I think the reason Graphisoft did not create a more refined or separate system for layout layers is because they want to include that in a more comprehensive layer management reform. I'm sure you'll agree that there are many other layer enhancements that would be very welcome. Among the most requested are the ability to keep layers from referenced or merged files separate and (related) organization by folders or subsets. Another is the ability to merge layers.
In the mean time my workaround is to name the layout layers such that they sort at the top of the list. I enable the Hide Locked Layers in Pop-Up Palettes preference (Work Environment-->Dialog Boxes and Palettes). Then for all my layout layer combinations I lock all the other layers and visa versa. That way when I'm working in the layouts I see only a short list of layers like in PlotMaker.
Regards,
Geoff Briggs
I & I Design, Seattle, USA
AC7-27, M1 Mac, OS 14.x