I've been looking around the documentation and the plugin files but couldn't find a way to add custom tools in this handy tools window.
I saw some doc for the Unreal plugin though and I guess it should be quite similar, but could you update the Unity plugin documentation so it's easier to implement our own custom tools?
In the mean time, you can create json files similar to the ones in $HFS/engine/tools for your own tools, store them in a directory, then in Unity on the Houdini Engine Tools window, select Add Shelf and point it to the directory.
