Likewise gold and hard assets will be demanded by foreigner in leiu of dollars. Let's say you want some fuel from abroad or a new TV. Just find a middleman, give him the gold, and get your TV. The government will declare that to be illegal, but that just means the middleman will get a bigger cut.
Bitcoin might be a game changer. It's intrinsic value represents the time, energy, and intellectual effort needed to produce it and maintain the system. Since there will be a finite amount coined, it will be scarce. It actually comes closer to real money than currency.
The only problem with bitcoin is that it is dependent on the internet for its survival. No internet or no internet access and bitcoin goes away. Gold and silver are independent of any other system. They stand on their own.