Would be willing to bet you could get either the SS or brass to work with the right bits and enough time spent on it. Brass would be WAY easier though.
If you try brass try to get something in the 300 grades as they are typically easier to machine (although they do have some lead content). High feed, low RPM (as low as you can get), and small cut depth.
The other thing you need to watch for with this kind of cut is that your material is coplanar (level) with the router through the cut. A relatively quick way to do the is level a spoil material before you mount the plate. That should help a lot. Could also use one of the auto level systems out there.
As far as bits go I doubt they are using a ball cutter for this. They are more expensive and since you are just going to be filling it anyway there isn’t much point. This would be pretty easy with standard stub style end-mills (short depth of cut tools typically designed for metal cutting). Or even V tip engraving tools if you can keep the leveling true through the cut. Obviously I’m bias on tooling companies so I’ll stay out of that one.
If there’s anything I can help with please let me know.
John Torrez
Think & Tinker / PreciseBits