Hey Hugh, engraving will be limited to minimal marking on glass so no 3d engraving sorry. Gf can handle curvature of the object asking as it fits in, using the camera to refocus.
As for 3d on other materials:
3d Engraving on glowforge is handled by using greyscale raster images not a 3d model but you could use a 3d model to generate the greyscale image. Glowforge plans to have 255 power settings, each corresponding to a shade of grey. (when I last heard word of it they were just receiving their new power supplies that will enable this, previously they only had on or off as a setting), I don’t think they have demoed 3D engraving yet, but that is the theory. I believe glowforge does not work with 3d files just 2d files(but you can export from most 3d programs to a 2d file format)