There is not one "MachineryBuildingBlocks" Object. Each Machine can have such an Object. The specification just defines a standardized BrowseName for that (like Devices does for specific functional groups) and states that the FolderType should be used. I do not see any need (nor mechanism) to do a more formal definition that would be reflected in the UaNodeSet without unreasonable and potentially breaking changes.

The issue is code generators need something in the NodeSet to trigger the generation of a constant for the MachineryBuildingBlocks BrowseName.
We do this with Properties by include unattached Variables. The same is needed for MachineryBuildingBlocks.

This is a real issue: the code generated for the MachineTool NodeSet assumes a constant is defined in the Machinery spec because the Namespace of the MachineryBuildingBlocks component is the Machinery specification. There is no work around that does not require hand editing auto-generated code or a complete rewrite of generators.

As general principle: if a specification defines a normative BrowseName like MachineryBuildingBlocks then that BrowseName must appear in the NodeSet.

We will add a template node to the NodeSet once it is defined in the Base Spec. See related Mantis Issue (0008640).