We want to remove (or at least hide in the interface) a built in test geometry.

How would we go about removing the shaderball test geometry and add our own version of it? Can this be done on a filesystem level or does it require using the hdk?

There's an HScript command to hide nodes ophide [www.sidefx.com]

Using the command will hide the node for the current Houdini session, so to make it work always you can setup a script called OPcustomizeunder HOUDINI_PATHwith the ophide command inside. For example, look at how it's setup with SideFXLabs [github.com]

Now if you make your version of this HDA then you should change the namespace, like company::testgeometry_shaderball

We got this hscript hiding technique to work which is awesome.

Is it possible to see the "code" for built in ops? Like shaderball for example. Grepping around in /Applications/Houdini shows that it might be .../Versions/20.5/Resources/houdini/usd/assets/shaderball/payload.usdc or a couple of other places - but what if we want to extend/modify the builtin test geometries? Or just see how they are built?

Shaderball and other test geometries are all Digital Assets. You can unlock them and dive inside their subnet and see how they are built. Usually their geo is saved under bgeo files and embedded in the HDA, you will notice this when you check the file path in the File SOPs used inside thses HDAs.

Regarding the usd files, they are saved here to be used as a reference when you use a test geometry in LOPs.