Hello Floriaan,
Thanks for the tip, Blender indeed helped me cleaning up the 3dm geometries for Capture use!
It took me a couple of days of experimentation, but the lollowing eventually worked: I exported in FBX from Rhino and then GLB from Blender. All geometries and materials were nicely preserved.
I tried all sort of scenarios within Rhino to be able to export directly to Capture (NURBS to meshes, exporting only meshes, exporting only polysurfaces) but had zero success. Using SketchUp as a mediator also didn't work, at least in this case. I guess I'll ask for FBX files from architects who use Rhino.
Thanks again!