The answer lies above, either the custom behavior shown a few years ago, or could wire it up with the InvokeCommandAction that is now part of Prism 5. Put that on the root element of your HierarchicalDataTemplate using a RelativeSource binding to point to the parent TreeViewItem as the source, EventTrigger catching the Expanded event, Command property pointing to your ViewModel command handler through a binding, and use the TriggerParameterPath property or CommandParameter property to pass along the context of what node was expanded.
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