BTW, FCStd-files with multiply bodies/parts already works. Here is a viewer link[1] for FreeCAD's built-in `ArchDetail.FCStd`[2] example model.
[0] https://github.com/kovacsv/Online3DViewer/issues/312
[1] https://3dviewer.net/#model=https://raw.githubusercontent.co... (ATTENTION! Huge CPU/RAM load on start!)
[2] https://github.com/FreeCAD/FreeCAD/tree/master/data/examples
[0] https://forum.freecad.org/viewtopic.php?t=41926&p=663399#p66...