Ok, I'm not going to say this is a good idea to implement because I don't think it would be fun at all.
But lets talk about the mechanics of the proposal. You're basically talking about instituting a money supply (which is distinct from total wealth since it only refers to hard currency). This would require currency to be an object which is the medium of exchange, and for the value of currency to float against that of goods. While those are not abhorrent additions in and of themselves, it would require far better AI for handling items before we really want dwarves messing around with currency (the current AI is awful).
The real problem with your proposal is you are advocating a fixed money supply. This leads to a permanent depression scenario as the workforce expands, because there simply isn't enough currency to carry out enough transactions for the economy to grow. If there is a money supply, the government needs to be able to expand and shrink the money supply. Specialized money merchants don't solve the problem - minting coins creates money. The question is whether there is too much or too little currency in the economy, and what that means for prices and exchange rates.
Of course, now we have to know something specific about the currency. Is a gold coin worth X dwarfbucks because it contains X dwarfbucks worth of gold? Or is it because the mayor fiats that value? These have rather different implications for the nature of the money supply and the effect of expanding or contracting it. Also note that if currency has value because of its content, that creates additional demand on its constituent metals beyond the demand of industry, which leads to inflated good prices and possibly industrial stagnation (if the price of a required good exceeds the value to the industry because people value it more for currency, the industry can't acquire raw materials).
I know, I've advocated an economics-related change elsewhere, but in that case it was distinct from government action and politics. Money supply theory is quite political and requires government action. For a reasonable money-supply theory of the time period, mercantilism, the consequences in-game would be that you couldn't normally trade for gold, silver, and possibly other metals with other civilizations (notably humans), because that's equivalent to trading away actual liquid assets.
Basically, I oppose this for 3 reasons:
1) The economic theory isn't sufficiently developed to create a good model in the game. If you could do so, you'd put the federal reserve board out of a job.
2) There's only one model which is sufficiently in-period to even contemplate, and it leads to less fun because you can't acquire some goods.
3) It would require a minor in economics to make plausibly intelligent decisions, which doesn't sound like fun for most people. Everybody understands how rarity and desirability relates to price at some intuitive level. Not many people understand how money supply relates to economic growth.