> establishing a secure communication channel with the person they were meeting with beforehand

How? Would that be illegal in itself in China?

> livestreaming their trip back to BBC HQ

Maybe doable if the local phone reception is good enough and not blocked (remember, the great firewall throttles VPNs)

> having an agent buy their tickets/hotels so it looked like a local was going instead of the BBC?

Is this in and of itself legal in China? Or is it one of those places where you have to register your passport number at the hotel?

Part of travelling as a journalist is doing so openly as much as possible - covert journalism runs the risk of being deemed espionage, which can result in a death sentence.