This is policed by the check-in agents, who are generally very good at HKG. Why would you want to risk it? If you are caught you will have to fork out a large amount of money to go on the flight you planned to be on, even assuming they let you go at all. You should also get blacklisted by whichever airline you tried it on with.

The contract terms for the ticket are clear. If they don't fit with your plans then buy a different ticket, but please don't do things like you suggest which might well result in the rules being tightened up even further thus making these tickets less attractive for the rest of us.