Instead of just having the coin stockpile be for unowned coins, why not have the option to make a "vault" room for owned coins? All coins that aren't being used go there. One could even make a "banking" job or a "banking" noble, depending on which is better suited, who will do all the coin hauling to and from rooms for the dwarves. Hell, give him his own (non player controlled) assistant haulers so it's more efficient. Owned coins would be put into coffers and sorted according to owner. If there are enough coffers in the room, the stacks will be split up so that there's a coffer full of a dwarf's personal coins for each dwarf.
Here's how I think it would work:
Urist McBuyer needs money.
Urist McBuyer works.
Urist McBuyer gets paid. The money goes immediately into the bank.
Urist McBuyer needs money. He requests some quickly from the bank (probably not requiring him to do anything - dwarves probably have a system or sometihng). A non-player-controlled hauler delivers it to his room, where he uses it as he sees fit.
Urist McBuyer spends some of his coin and saves the rest in his room for later on his "break" time.
If your fort gets too large, you can even implement a system somehow where you have dozens upon dozens of coffers in your room, and like stated above, each one belongs to a personal dwarf - but no coffers are used in personal rooms. The banker would keep track of transactions, the dwarves would use fake money, and just like the gold standard, each dwarf would have a certain amount of coins in their name that they never get their hands on. This would be more efficient, since all a dwarf needs to do is request fake money from the bank, and then the same amount of coins are moved from one coffer to another, which is about three squares away. The dwarf never needs to use his coins, just get fake money of the same amount from the bank, and the bank will keep track of how much money each dwarf has. Very efficient.
Having extra nobles or non-player-controlled haulers that come with the tax collector makes sure you don't have to have too many dedicated haulers.
Any thoughts?