Edit: Of course, I decided to reply without reading anything...looks like this idea has been mentioned (room objects). However, I will say this was my first intuition, after reading just a couple posts in this thread, none of which mentioned 'room objects'. So...
I haven't read thru the whole thread, so pardon if this idea has been mentioned, but what about this:
Sub-layouts appear as objects in the master layout. Their size is determined by the dimensions specified in the sub-layout, and cannot be adjusted in the master layout. However, they can be moved around and snapped together, just like other objects.
If you double click on one of these 'sub-layout' objects, it opens in a new tab for editing. When viewing them in the master layout, you see a static visual of the current state of the sub-layout, which updates whenever you change it.
Obviously, there are some limitations. One that comes to mind is that sub-layout cannot have their own event sheets, properties (other than size), or effects. Instead, these have to be applied to the master layout, and then inherited by the sub-layouts.