In spite of the fact that I can't offer any more examples than Nanning, my gut tells me that there must be more Nannings out there.

Basically, any city with an airport bus/train is ostensibly fair game, but if you've never taken the bus/train, you would likely be unaware of the existence of such services.

As an aside, in the HK case, in addition to the hex, it's also possible to check-in (including luggage) on ferries from Macau, Shekou, zhuhai, and a few other places.