I don’t mean to imply anything (I aspire to state things exactly as they are, or indicate I don’t have a definitive answer.)
In this case, I have never used a Mac with an eGPU nor do I believe we’re doing any testing with that setup. My amateur understanding of the eGPU functionality was that it, for all intents and purposes, behaves like a discreet graphics adapter and thus any computer using one would behave as a computer with a normal discreet card. To this end, I wouldn’t expect that SketchUp would have to do ANYTHING to make it work. If this is incorrect then it makes the eGPU seem significantly less useful.
As far as SketchUp and a discreet graphics card, SketchUp will lean on a 3D accelerated card for handling any OpenGL operations before it uses the system CPU.
While my first response about LayOut would be that it’s an OpenGL application… that might only apply to the embedded SketchUp functionality. I’ll @colin on this one, he knows a lot about Macs and may have more insight as well.