I honestly think it could work the exact same way in a fantasy world as in a modern one. I personally wouldn't be handing these out as achievement rewards, but rather as markers of devotion. The PCs won't be given these except by an organization that they've associated themselves with, and become true members of. I think they might be able to function roughly like any "in-group" item, allowing you to have premade contacts within a city, people who will act favorably and also those who won't, simply by the fact that you have the coin you do.