Using Toolbox setup it would appear that you can assign a custom property to an individual part,
"Once you create a property, you can apply it to an entire standard, category, or type, or an individual component. For text configuration-specific properties, you can type values directly in the configurations table. The custom properties you assign appear in the component PropertyManager. When using the component, Toolbox users can enter custom property values in the PropertyManager"
Every time I apply it, the whole tree gets updated. I just want a plain Stainless Steel screw, but get plated versions.