With the popularity of DIY mechanical keyboards I have to imagine that at some point some dedicated individual(/crazy person) has machined individual keycaps from stainless or aluminum.

There are titanium, steel and zinc anode keycaps floating around. I know because I've owned some for about a decade :)

You can probably replace every component on a keyboard, today, with metal with the exceptions being limited to the switches and PCB.

I've reduced, not eliminated. Custom keyboard, titanium case and wooden keycaps. Plastic still in switches, wires, and pcb. Normal mouse, I've tried alternatives but nothing fits my expectation.

I suppose I should have known the costom keyboard community would provide on that front! Wood keycaps sound nice, I will have to look into that.