As of git commit 496df343
, Arch & BIM workbenches now have a new "Reference" tool/object that can import/link the geometry of any object that has a shape (including BuildingParts) from another document. The linked document is scanned for changes every minute (that interval can be changed in the preferences) and if changed, the reference object is marked for recompute (no automatic merge, on purpose). In context menu, you have options to force reload or open the linked file.
This is a VERY simple implementation, it doesn't open the linked file in FreeCAD, it just swallows the needed brep data from it. This is a quite different path than Assembly3, so I chose not to use it, at least for now. The idea here is not to have fine control over the linked objects, but rather be able to work with very large files (say, whole buildings or building levels).
In the future, I'll try to add one more level to this, which is to store and import the inventor representation only. But this should already allow us to work with significantly larger models.