From the First Opium War until WWII, more than 80 treaty ports were created in China. However, in 1943, the Allies agreed to give up their treaty ports. Following the communist takeover in 1949, the last vestigaes of the treaty port system disappeared (unless you count the New Territories, which I don't).
So, with a PoD on or later 1 January 1900, make the treaty ports last longer--at least until the big wave of decolonization in the 1960s.