So... A regular brass plate with a pc spacer? I dont think that's going to feel very different from typing on brass. It'll actually be stiffer.
Also, the PC part being only 3mm is going to leave a ~.5mm gap between it and the pcb, so it'll probably rattle and create more noise unless its adhered to the brass plate.... But again, you're just making a stiffer plate. Resonance is still going to be an issue because its not like the PC is going to dampen vibrations.