I.e., once a subassembly is attached to a build, it can still be treated as one part with one attachment point. For instance, if someone adds a "cockpit" subassembly to a build, it appears in a list of subassemblies that the current build uses. The user should then be able to select that entire assembly by clicking on the list, and move/rotate/delete it like a single part. This could probably most easily be done using a new attribute on the Part tag, e.g. subAssemblyName, or you could have a new tag SubAssembly that contains Parts.