If you code the withdrawl/check of the banker to use the multiple currency consumption, then you can tell it to only give out bank checks when you want to make checks. With this it will make the biggest types of checks possible, and then you can cash those for the biggest type of coins that your system has.
I know its a little dirty trick, but other than that you can code an NPC which will convert it (all you have to do is tell it to consume all of your coins or checks, and replace it with change equal to the amount you consumed).
I know its a little dirty trick, but other than that you can code an NPC which will convert it (all you have to do is tell it to consume all of your coins or checks, and replace it with change equal to the amount you consumed).