First here is the documentation
https://www.sidefx.com/docs/houdini20.0/shelf/config_file [www.sidefx.com]
Graham, I then took your code, and modified this part, by adding <contextNetType>VOP</contextNetType>.
<toolMenuContext name="network">
<contextOpType>table/operator</contextOpType>
<contextNetType>VOP</contextNetType>
</toolMenuContext>
Also for someone looking for the topic, there is relevant discussion
https://www.sidefx.com/forum/topic/40864/?page=1#post-183147 [www.sidefx.com]
Thank you for help.