Hello Polycount, dear EQ,
while previewing my model in the marmoset viewer (just before exporting), I clicked through the menu options and found the developer tools.
These are basically the same as in every browser and lets me, for example, hide different UI elements, change the thickness of the loading bar and probably change the icon one has to click to load the scene in the webbrowser.
Is there any way to add these temporary changes to the model viewer instance I would be using?