Eventually, currency bases should be part of the procedural generation. It's possible that there's more in the game already than most players use; the old custom of making a set of "commemorative coins" for your vault each year has largely died out. IIRC these had procedurally generated designs, or at least symbols.
But we'll need some sort of generic name for ☼, when each word has its own; calling them either "Urists" or "Turtles" would be my preference. There's even some real-world precedence for turtles; it was one of the shapes that tael "coins" were minted in.
Note that DF's current currency is sort of a weird hybrid; if I'm reading things correctly, a DF coin is worth 10x its base metal value consistently. They are also fundamentally a *volume* measure; a DF coin is physically 1/500 of a bar (or a bit less if there's some loss), and a bar is 600 cm^3. So, a DF coin is about 1.2 cm^3. For comparison, a 1 troy ounce pure gold coin would be about 1.61 cm^3, and a "crown gold" coin of 1 troy ounce pure (about 1.09 troy ounces total, for instance a Krugerrand) would be more like 1.75 cm^3.
IIRC in the old currency system, gold, silver, and copper coins (of 1/500 bar) were legally "coinage", while those of other metals were technically what would be called "bullion rounds" in modern terms. With relative material values of 30, 10, and 2 respectively, even with what amounts to 10x seigniorage this didn't give a very useful currency system. To buy a bar of iron (relative value 10, same as silver) at par would require 50 silver coins, 250 copper coins, or just under 17 gold coins.
The other thing this brings to light is that gold is almost certainly seriously undervalued. A gold bar is valued at 3x an iron or silver bar, but in contrast to most world cultures where that would be a weight (mass) measure, in DF "bars" are volume measures. So a gold bar weighs (using DF "solid density" figures) 1.84x a silver one, which means that on a weight basis gold is only about 1.63x as valuable as silver.
Interestingly, one interpretation of the 10x "seigniorage" is that is in effect the quality modifier for the coins (as coins don't get normal item quality modifiers). That's closest to "Masterful", at 12x, and the symbol for "Masterful" is... ☼. In other words, Masterful is literally the point at which your crafts are a more valuable use of a bar of metal than making coins out of it. Kind of neat.