@karhide and @35mm - Correct.
The plug-in effect I am testing has some of its features hidden from its main GUI, as these will eventually be hardware controlled (and therefore do not need to be represented in the GUI).
In Reaper, there is a simple button to switch the GUI on and off (which is what I am now using for this purpose), but as a fully paid-up Sonar user I wanted to continue using it if it could do what I wanted. Unfortunately, it seems I can't.
Here is an example of what I mean using a Waves plug-in (although in this case, all parameters are implemented in both): no GUI on the left, GUI on the right...