My best (stupid) guess is weapons. US gov has a ton of weapons. Already sells tons of weapons. Now the prices will always remain stable. Other countries would love to know that 30 million usd would always be able to buy an f15. Other countries declaring war will increase the value of the USD, as buying weapons from us government will decrease amount of money in circulation.
I mean the hack which leads to the best result would be pegging it to the Euro, Pound or Yen as a strong currency controlled by a US ally
That said, backing it with recaptured pollutants or something would be interesting as although it has no inherent value, that’s no different to how it is currently and it’d be interesting to see the mechanics of how people act