The UST missing an interest payment seems to be a ultra unlikely event. As I said in the past people seem to be unaware that the Fed would step in to prevent such outcome, invoking section 13(3). They would not even need collateral for the loan since it would essentially a purchase of a UST. This could be complicated by some law that prevents the Fed from directly lending to the UST(But I'm not confident of that) but they can bypass this issue by lending to an LLC(Maiden Laine No n) which in turn lends to the UST