Unless you're in transit, I don't think you'll be able to get to either of the CX lounge complexes on arrival. At HKG, you deplane "downstairs" and are funneled straight through to Immigration at the main terminal. To get to the lounges, you have to go upstairs first (then back down!), and to get upstairs, you have to go through an intermediate security screen, where they want your same-day onward ticket and boarding pass. No ticky, no pass.

But I'll bet you can get an early check-in at your hotel.

And on departure, you can come as early as you wish -- the lounges are open throughout.