Yeah. That was me
https://sourceforge.net/p/sweethome3d/feature-requests/1084/
I also wrote a working patch attached to the feature request, that also included adding a "separator" character in the "o" line. And I wrote a patch for the blender legacy python obj importer so that objects with the seperator were grouped into collections in blender by elevation, levels and groups. Walls, ceilings and floors were grouped in their own blender collections too. Finally, groups were assigned to the blender objects as vertex groups.
I didn't touch the SH3D import code though - I'm not a Java dev, so just doing the exporter was hard enough.
If you go here you can see a screengrab of the blender patch hierarchy result:
https://developer.blender.org/D16095
The blender guy wanted to re-do it in the C++ importer... <drums fingers>
