I have never heard that any collateral has been pledged. The FED was established almost 110 years. Until the past few years, the subject of collateral never came up. This is not your father's oldsmobile.
Unfortunately this was not a worthwhile debate. The question posed was not very well answered. Laurence White is a self-admitted oddball and clearly does not understand why the gold standard failed and can not be resurrected. Even contemplating the idea that Crypto could replace Sovereign issued currency and somehow make Central Banking unnecessary seems a total waste of time. White failed to convince anyone not already convinced that it is otherwise. On the other hand, to use an interjection taught to us by economists, Fred Mishkin got his Ph.D. in the seventies and appears not to have updated his knowledge of Money and Central Banking much. Far better arguments are available than the ones he used.