There isn't really much you can do. I don't know how many people have experimented with sanding them rough again, but I reckon that would be one of the only ways of de-shining them. This is also why shiny key caps is often of low value compared to NIB keys.
alternatively you could maybe also use some adhesive film/foil with a rough surface, cut it to size and add it to the top of each key... or clear coat them in something that would give it a rough texture.
There is possibilities, but you will have to experiment.