Where can I find information about Chinese security policies with regard to air cargo?

I guess as a start you could contact the Chinese consulate.

I was going to say what @rojo said. The nearest Chinese Consulate or their Embassy in Washington. You might also contact the US State Department to see what advise they have that the Chinese might prefer not to share.

In general, these regulations are not publicly accessible (for good reason!).

Depending on your need for that information, go check with a good freight forwarder, or airline. They will be able to tell you all you need to know as a shipper or consignee of air cargo.

The China Law Blog maybe?

Here’s what Wikipedia says about it: