I say it should stay as a brick. If you want to use them for a currency, make them add a variable "Gold Coins" to you and do the rest through events.
Although it would be cool to have a bot drop these as "loot".
Meh, both would be useful.
But then it wouldn't be physically there and it would be as lame as RP money. Also, unlike RP money, coins will have a limit system. With RP money the amount you can have is infinite. With these gold coins you can only have as much as your bag and chest can hold.
That and like you said, it would be cool to have bots drop these coins as loot.
*On bot death - drop random coin - 10, 20, 50, 100
*On bot death - drop coin - 10