Could you mux the data of the coins?

If you have a stack of gold containing 5 uuid and mix it with another stack of 3 uuid but only support 5 uuid per stack. Then calculate proportional weightings which 5 you store.

You could probably find a way to try and wash the money but I think it could be done with right heuristics to catch a good majority of use cases but maybe not all.

I just want to note that I only started paying attention to cryptography after I left UO and got into Bitcoin, which sounds like an NFT-grifting MMO supervillain origin story - I know - but I sure did learn a lot about merkle trees and solving interesting social problems.

Are 1 UO gold coins even individually addressable?

No, stacks are stacks. They’re an object id with a quantity counter. Each stack, however, was capable of holding its own separate script and objvars; there were event triggers to handle special cases like “onStackAdd” and such.