I think what I’m looking for is sort of what you describe, but without the need for Layout. If there were a plug-in allowing hybrid-rendered SketchUp scenes to be imported into Autocad, then they would be treated just like XREFs, attached PDFs, or other externally referenced files.
The main hindrances to this sort of plug-in would likely be that Sketchup has it’s own Layout to support (sort of a CAD program) whereas Bricscad and Autocad have their own 3d modeling programs (not as simple as Sketchup) to support as well. Perhaps neither has a motivation to interface with the other. But maybe a third party could make it happen. I’m talking myself into it…