A couple ideas, well if I understood your problem.
In a .dentalCAD scene you can go top right and save .scaninfo. This might do the trick, your client only needs to open the .dentalproject with "DentalCADapp.exe" and not the scene that you have created, he'll probably be able to see the margin as a green line and you could save your design in stl that he can import as generic. You can also open .constructioninfo or .scaninfo in exocad viewer. It's quite technical but possible.
For exemple here, I just started a scene and set my margin line then saved and closed.
My dental cad automatically saves the .scaninfo but you can do it by hand.
I opened it in exoviewer 3D, it asks to open or not the .dentalproject, say yes and this is what I get. You can see the margin line.
View attachment 44850
Hope it helps.
Oh and don't change names of files or their coordinates or it won't work.