Did you ever figure this one out? this is the top (and only) thread when googling this.
the commands @dive linked appears to be QT based so likely is for custom tools: hou.qt
there is info here on adding menus to houdini’s main menu bar, but it’s not python : Customize menus,
Hint on where to put your tool
Paul Ambrosiussen 9 days ago
You should make a Houdini package where you distribute all your functionality in!
How you call on your functionality highly depends on where you expose it
Does it affect nodes in network editor? OPmenu.
Does it create things in your viewport? Shelf. Etc etc
There’s a list of the different menu’s mentioned here