If there is to be a currency, it will have to be player driven and not relying so much on the dev like developing a central bank. Maybe could be developed by the players and run by the players until it collapses, but no actual in game code that automatically does any depositing or converting.
Currencies add a new layer of complexity to the game, but must be agreed and used by the players and not enforced by the devs. Coinage equipment with stamps can be developed but the use and system of coins must be enforced and used by the players. Even if there was a "Banker's Table" which help keep track of how much of each XX,YY,ZZ Village coins are in circulation in game would help... but not necessary.
Guaranteeing value of coins is silly... in the sense that you remove a lot of game play aspect from the game. Then villages who topple a large faction or empire will be able to instantly demolish if not heavily reduce the worth of the faction's value to the point where it is only worth as much as its face value. Villages will have to develop and build large networks and manage their minting and coin supplies. If they are giving too much coins out to purchase supplies for a war, it will probably drop its value by a lot.
IMO If people want to use a coin system, its a risk you have to take. But the rewards are great as you can get a lot more stuff in shorter time. For example lets just say that a "Gumby Coins" is one of the most popular used currency because it is stable, worth much more than its face value, easy to transport, circulated throughout many villages, "Gumby" faction has a lot of security with military that ensures security of their currency, and lets say it is backed up by a gold standard (cuz Fiat is a nightmare... imo) But "Plumbers" Faction knocked out Gumby Faction and declared all of Gumby's Coins worth nothing more than its metal value. This shafts anyone who is trying to exchange their worthless coins for new Plumber Coins as the Gumby Coins are pretty much dirt. (assuming we aren't gonna be minting rare metal coins and using the face value of the coin as the trade value or everyone would just horde the gold and not use it to trade but to make their own stuff which removes currency from circulation. But if you use bronze coins, copper, tin, or iron as coins backed up by silver and gold stored in a vault. Then the only way to remove coin in circulation is for the players to buy or return the coins to the main village in exchange for gold or goods equal to the value of the coins backed up by the gold/silver standard... if we are gonna use that kind of standard... BUT i understand there are a lot of problems with using a silver or gold standard... there is really no perfect coin system.)
Or hell.... Gumby could offer 1:2 raito for Plumber Coins and buy out the whole circulation of those coins then go knock on Plumber's faction to give over their gold equal to the amount of coins they have... or prove they have the gold to back up the coins they have... If there was too much coins minted and owned by Gumby, then Plumbers would be screwed and people will realize that the worth of that currency is worth nothing but the metal used to produce it and people would have no confidence in using it to trade with.
So much more fun... villages further away from any major factions would stick with bartering as their coins will be worth nothing more than its face value and word of mouth.
i dont know where i am going... just rambling and thinking this would add some interesting economic and diplomatic negotiations

Not to mention could give meaning to wars with factions over economic power and currency strength :3 Unless we are all gonna accept 1 universal standard of some village's coinage.
Summary:
Stamp for custom coins yes!!
Central bank that automatically exchanges stuff developed by dev no!
currency developed by dev, No!
currency handled and attempted by players yes!!!