If you make a keycap out of any material of your choice then it will work. The keycap is not connected electrically. Unless you are making keycaps with LEDs inside them.
Would you be interested in making keycaps that are unprinted or printed on the side?
Then you could experiment with putting something interesting on top of the keycap, such as suede, felt, satin, or that gel stuff that is in those Magellen shoe insole thingamajigs, etc. You could make something really luxurious!
And please tell me if you come up with something that really works out well. I am really interested in this kind of thing.