Hello Nicholas,
Now this is not the "clean" solution by far, but just to get started, how about you make the GUI with a nice GUI library - no matter if IronPython, Python, or something else - and save the data from the GUI in a text file, that you observe from Grasshopper? So whenever that text file changes, Grasshopper updates.
The KUKA|prc GUI is done using WPF, which you can also design in a graphic editor within Visual Studio, so that is of course also an option.
Best,
Johannes