Another way that GUI in VUO could work is for users to publish ports they want to hook to GUI.
Another app (a VUO interface builder?), or maybe a built in feature, could cycle through the published ports and programmatically spawn GUI for published ports.
It could be that the user adds a specific key to published name or something to control whether a knob, slider, etc, is created…or they could be presented with a set of options that they then choose from via an interface.
If you look at the sample code for Apple’s QCTV project, and I think QC Performer (or some name like that), you will see a very simple concept along these lines. The GUI is programmatically spawned by the QCTV host app, according to published port types.
That said, I think there is a strong use case for exposing a way to see something like a knob or slider (and other GUI objects) as a node itself, within the VUO editor.