Well, first of all I don't actually intend to use this hammer to smash things to bits, for that a good old steel hammer is still the best choice, if you're in a non-explosive/flammable atmosphere at least. But for softer things I use aluminium scraps or MDF as stated before, but even with aluminium you got to be careful which alloy you use, some are pretty damn hard. Additionally the entire point of a copper hammer is that it's softer than the material you're hitting.
And about press platen, actually you absolutely do not want brittle things for those. Otherwise you might end up with some interesting problems when you run into non-flat press books.