It would probably be overvalued at first and take a little while to decline. We'd destroy a lot of HIVE creating HBD to dump on to the market to push its value down to the new value of USD, which could be good for the price of HIVE (over and above that the price of HIVE would likely go up in USD terms just due to the USD flash crash itself).

If it seemed like USD had utterly failed as a currency, we'd probably switch the peg to something else such as a basket of commodities, after some time to debate what the new peg should be.