But who is going to hold the bag?

Who will be that wretched fellow appointed to keep up the rates by selling precious assets if panic sell-offs start all of a sudden? After all, the government which has set the fixed exchange rate for a currency backed up by some hard asset (say, gold) also happens to print that currency. So if it keeps its monetary policy lean and mean, it won't have to face insurmountable difficulties with supporting the exchange rate as it deems appropriate. Besides, I heavily suspect that it is next to impossible (if not outright impossible) with decentralized currencies as well as pretty meaningless, and still more so with currencies which are hard capped (the cap itself would serve as a substitute for a "back-up")