Allow unmodifiable/parameterizable Sub Assemblies

It would be awesome to create sub assemblies that cannot be separated in Craft editor. These sub assemblies should be editable individually but behave like a grouped part when imported.
This would allow to create launch vehicles and compose them and make minor adjustments without interfering with the full rocket design (sure, some edits could change offsets and this would cause conflicts).
But the parameters should be modifiable, like fuel volume or dead weight, but no split nor resize.
I envision launch vehicles being composed like real ones, and payloads being added without interfering with the staging order or risking to drag a wrong part and mess with something.