A queue to cash out would be fine and the timelock is relatively small (especially compared to 13 week powerdown). It would also be possible for the Polycub vault to try and buy HBD from other exchanges if it become illiquid like that, but that's a pretty centralized solution that would only be done by hand in case of 'emergency'. (Assuming we can call users needing to wait a few days an 'emergency'.) Not exactly a typical bank run. The vault won't be loaning that HBD out to anyone, so it will all be right where it's supposed to be.
Those Emergency Powers, muh confidence.