I should mention that I managed to get FreeCAD to manage unit systems per-project. Several months ago, in fact.
I haven't attempted to offer it, however, because even though I can shift the metrics internally, there's not any good event I can find to fire to cause the change to propagate when switching view panes. This is especially noticeable when opening a project file.
It would be nice if there was something for this. For one thing, even with the current every-project-the-same setup I've seen the units changes lag on recent production releases.