It looks like one of my earlier replies got blocked, perhaps due to an insertion of a weblink to an newspaper article.
I agree with you folks for sure the HK outbound flight will be safer than the return leg, but in this newspaper article it reported a study was done by an SK university on a flight from Milan to Seoul. In short, a lady was diagnosed with CV19 on her 14th (last) day in quarantine. Their conclusion through analysis was she was mostly likely caught the virus when she visited the plane's toilets. She wore the mask the whole time during the flight, but strangely not when she went to the toilet.
The newspaper said the toilet was shared by many passengers sitting nearby, including an asymptomatic patient. The problem with plane's toilet, at least to me anyway, is when you flush it there is a massive backdraft of air from the system which has all sorts of unpleasant stuff in it, virus included.
Unless you can hold your toilet break for 14hours, I suggest you keep the visit to a minimum.
And of course wear the mask when you go!