Like Jeff said, your only option for replacing gold is to either replace every vendor with something like a custom stone that you can purchase goods through, or to make gold so common that it doesn't matter anymore. Make it so that one of your new currency could be traded for 1 million gold.
It depends what end you are trying to get to. If you are trying to modify the whole economy just for the fancy look of it, you will run into issues with things like bankcheck's, housing, insurance, etc. If it is just a way to make gold look fancy, I think you are going to spend a lot of time and end up frustrated. If you don't like how worthless gold is in a typical UO server economy, you would better spend your time modifying all the lootpacks and vendor sell prices to make gold hard to come by.
perhaps look at adding a secondary currency to all the lootpacks along side the gold if you are just looking for something new to play with.