[thaumasiotes]

So, in the historic situation in which a farmer's business is all conducted in copper (or in kind) and taxes are assessed in silver, would you consider silver a fiat currency?

A fiat currency is fiat because it gives you no claim to anything. A pound of silver gives you a claim to a pound of silver. A US dollar used to give you a claim to a certain weight of gold. Today the price of gold is uncontrolled.

(Side note: owing to the special nature of facio/fio, using "fiat" doesn't necessarily require an agent. The verb is indeed the passive form of "facio" [do/make], but it's also the standard way to express [happen/occur].)