If you are checking in bags, you could check in early at the city centre check-ins at Kowloon and HK Central. You need to buy an airport express ticket to get into the barrier, but you can always travel to the airport later (or even take a taxi later if you don't mind throwing away the HK$100 ticket).

When this has happened to me - and HK is the main place it happens - the flights have been chocker and either an OpUp arises or they think about asking (but don't actually get round to it) about shifting my seat to keep families together.