Sure - it's all bookable over the phone whether under or over 24 hours in HKG. Usually the website does allow transits of under 24 hours so it should have been bookable online. The trick in that case might be to try it as a multi-city booking to see if it lists all the options up to 24 hours after landing. The error I get when attempting a stop-over is "BA do not fly this route" even though if I change the times to a transit they suddenly do fly that route