How could the US Central Bank counter BRICS?

With BRICS possibly announcing a primary medium of trade that is not going to be the USD, how could the US central bank counter this by backing the USD with BTC? How would that work? Would they say any new USDs printed would be backed by BTC stored? How do they back-reserve all the USD already in circulation? Put aside how they would store and secure it. Just trying to understand how the USD becomes backed by BTC when there is so much USD already in circulation.