The worst-case scenario (large drop in demand coupled with large increase in supply of US treasuries) would probably only be triggered if China dumped a lot of treasuries on the market at once. Say, for argument's sake, 50% of their holdings in one week. This would signal US treasury weakness and would flood the market with a ton of extra US treasuries.
They are currently selling treasuries off at the rate of ~1% of their total holdings per month. So I assume that fear over an uncertain stock market + investors looking for a good opportunity to pick up cheap US treasuries will provide enough demand to soak up the new supply of treasuries that are trickling into the market.
