Cloning a node is a high-level operation, implemented in subject hierarchy plugins. It involves cloning/copying node references and various node properties. For example, to clone a volume node, you need to deep-copy its image data, clone at least its display nodes and copy some of its node references.
There are no subject hierarchy plugins for low-level nodes that are mostly just implementation details. Volume rendering property (and display nodes in general) would fall into the low-level nodes category, which you would not want to pollute the data tree.
Maybe a “copy from” volume rendering property button somewhere in volume rendering module could be more appropriate solution if cloning turns out to be needed frequently.