But depending on where you end up, a hose may still not be practical at the end of a ride. Also at the end of a summer ride wearing body armour and full face helmet, you might want to sit in an air conditioned van for an hour rather than look for a hose.
LCSD? I'm not aware of any facilities near rides. If you meant AFCD, most of the good trails in HK are illegal and there is constant low level tension between the AFCD and mountain bikers.
The public toilet hose might be a workable idea, but it's also likely to piss off the neigbours- the amound of mud generated by a bike really stands out, and when done by a Westerner is less acceptable than when done by a local who's lived in the area all their life. And personally, I want as little to do with public toilets as possible.
I've spent a decade thinking about this, there are no 'ideal' solutions like someone would have in a country with front yards.