I have exactly the same thought and have expressed in at least two threads before. I think to make it work there are several possibilities.
One is that Nu installs bots on exchanges that trade commodities or ETFs. So a user who has an account on these exchanges can use commodities or ETFs to buy pegged Nu products from the bots, and vice versa for selling. The reserve stays on the exchange in bots' accounts, just like how USD is dealt with today. Nu has to work with these exchanges and I'm sure these traditional exchanges care about regulations much more than cryptoexchanges.
Another possibilitie is trading using proxies just like we mostly trade NBT/BTC today instead of NBT/USD (USD being the peg target). The user can use USD to buy/sell Nu products pegged to goods or ETFs anywhere but the price is set by the bots according to market rates. When users trade this way, the bot executes an immediate buy/sell of the commodity on the traditional exchange using its own USD/reserve. If the commodiy/ETF exchanges also trade BTC or NBT, the proxy can be BTC or NBT, too, instead of USD. As has been discussed elsewhere, trading using proxies is getting exposed to proxy price changes.
Seeing how hard to sell NBT in the NBTUSD market, it could be even harder to do in the NBTcommodity market for most people. I guess the shortest path is getting an ETF trading exchange to add BTC or NBT and go in the proxy route.