My family are arriving soon to visit in Beijing and they only have a single entry visa

they want to make a trip to Hong Kong while they are here. So their single entry will be used on arrival and we were wondering if we could use the 72hr free transit to manage the problem. If we make sure their Hong Kong to Beijing flight arrives no more than 72hrs before their (already booked) Beijing to London flight can we use the 72hr free transit without a problem?

Thanks,
Andy.