Thank you for your question. This is a tricky one. When NodeData::Read is emitted, Cinema 4D is currently reading a file and in the process of deserializing your node; so it indeed is not yet 'fully instantiated', as that is the whole point of the method. You are here not meant to manipulate the scene graph (the node is not yet attached), but read and store data on your plugin hook. I.e., you are supposed to do something with your MyMaterialData and not the GeListNode/BaseMaterial which is being passed in as the first argument of Read.

The common workflow is to read data from the HyperFile which is passed in and store it on your NodeData instance. Passed in is also the important version number (which is called for some odd reason disklevel) of your HyperFile container.

As always, you are bound to the main thread for scene graph manipulations. So, NodeData::Init is not a good candidate when wanting to poke around in the scene graph, as it can be called off main thread and on dettached dummy nodes. A good approach is to hook into NodeData::Message and listen for MSG_DOCUMENTINFO to catch a document just having been loaded. This is for your specific case of wanting to manipulate nodes which have been deserialized. If you want to manipulate node instantiation, MSG_MENUPREPARE is a god choice. But it is only called for nodes which have been instantiated from direct user interactions such as a menu click. So, MSG_MENUPREPARE is not being emitted for nodes loaded from a file.

The drill to do what you want to do would be:

1. [optional] Overwrite NodeData::Read/Write/CopyTo to read, write, and copy custom data, and possibly use/detect different disk levels to catch different versions of a node type being loaded. You would have to match this with incrementing the disk level in your RegisterMaterialPlugin call, so that there are old and new versions of your material type out there.
2. Hook into the document being loaded, check the scene state, and possible data left behind for you by NodeData::Read and ::Init.
3. Manipulate the scene graph to your liking.

Cheers,
Ferdinand