Oops, you are probably right Eu reg261 would not apply to a HKG-BKK flight, however my comment that this may be a cancellation remains. Check CX's (or whatever) coc for cancellation provisions.
I don't think HKG courts would award more than reasonable expenses, but I am a (lot) less familiar with their jurisprudence. However any airline faced with this prospect would fight any precedence to the end.