Ah, if I recall, check in is 48 hours, or 24, at the very least. When in J/F, I think they'll check in your bags early or able to interline it, but I'm not sure. I used to do this all the time when I have those early morning flights (I generally try to avoid them, but sometimes it's a necessary evil) and just head into HKG after dinner the night before. While entering security, my BP would get flagged, but those manning security would always just override it to allow me entry no questions asked. I must have done this at least 10 times.

I basically just stayed in the Pier F's day suites, then headed to one of those 24 hour PP lounges. Then, at around 4 am, headed back towards a CX lounge for some coffee and maybe breakfast before my flight. I find this preferable over leaving HK at 5 am to make a 7 am flight.