Actually I don't think it *requires* making a breaking change. It simply requires making everyone who trades in your "gold coins" a little more rigorous about their behavior. Either that or making the client a little more explicit about when it creates merging transactions.
An example of rigorous would be, the person who bought "gold coins" would have them transfered into a virgin bitcoin address. In your wallet, you could have you client tag that address as "non-merging". Everyone who received them would also do the same thing. In that case, when they are redeemed they would be "untainted".
Should someone inadvertently taint the gold with other coins, they would have to continue trading/redeeming all the tainted coins in one block. In effect, you have to lose the traditional value of the tainting coins to preserve the "gold value" of the tainted ones.
I initially thought the same thing. I also noticed that you'd probably want to take into consideration non-fungible vs non-fungible and divisible vs indivisible commodities.