Frankly, I expected FreeCAD BIM to base its modeling features on IFC files rather than FCStd zipped FreeCAD-specific XML files. But at least there should be a documented bidirectional mapping between FC objects and IFC ones.
Next, there should be some IFC extension that allows you to define more fields, like parametric alignment. This extension should be thoroughly documented and versioned independently of FreeCAD repository. Apparently, custom properties are already supported by IFC.е
After basic roundtripping works in FreeCAD, there should be a push to synchronise custom property sets: Blender BIM should be the easiest to go for. Then, there's revit open source exporter. And of course, IFC itself.
Anyway, this is an off-topic here.