I had an idea to make RAISED letters in the molds of the keycaps once. Seemed like an awesome idea, and many thought so. So I got some quotes, and was $500 for each new unique key mold. Pretty excessive. I think unfortunately Elrick, your idea would be similarly cost prohibitive as it would also require a new mold for every unique key. Though I suppose you could also use an 'extreme' engraved approach in theory as well. Not sure if the resulting legend knock out would be as clean. Should be a lot less expensive though.
Other downside is stuck with stencil like font as some letters have a solid space inside, like A, R, etc...
An interesting idea for sure.