As stated above, get rid of your luggage at the in-town check-in; they'll have no problem with checking you in early on in the day and issuing a BP. The check-in at the airport generally opens only three hours before the earliest departure.

As for going airside several hours before your departure, absolutely no problem, provided you've got a BP valid for a departure that day.

For anyone on a < 24 hours stopover in HKG and flying onward with a OW carrier, one useful trick I've learned is to go get your onward BP from the transit desk, rather than clearing immigration, use CX's The Wing to freshen up and then clear immigration and retrieve your baggage. If you're arriving on CX then you're entitled to use its Arrivals Lounge in HKG, so no need for the above.