Bronze takes patina really well, so you could pre-age it in minutes.
Gold/silver casting wouldn't work (probably) as you usually give a casting metal about 150oC (or more) super heat which will partly re-flow the first "shot". Platinum would work for a first cast as its melting point is way up there, but you are then talking hundreds per cap.
A much more sensible way of doing it would be to use standard inlay methods - cast a silver key, carve out the lettering and seat gold wire in the recess or, even easier, flow gold solder in.
This works with other metals too, brass/bronze/copper with silver or gold lettering - just remember that where you strike the key you will ware the patina away - there's a rather famous bronze statue (of the female persuasion) who has taken on a beautiful even deep patina, except her buttocks which look like they came out the foundry yesterday...
Any other thoughts...
Ohh I have an idea for a single run (I'll be making silver caps over, and over, and over, and...) which will be horribly expensive - other than that I don't want to spoil the surprise.