I'm going to second this, get yourself a flight to Tokyo and then return as the JAL lounges are arguably better than the Cathay ones, and far more interesting (you can even go via Tapei if you really wish) - and I don't consider there too much difference between NRT and HND.

My personal advice if you really want to "maximise" your experience book onto the 1am CX524 flight to Tokyo (NRT), then you'd get almost 20 hours in the lounge without any strict questions (beyond your sanity for arriving so early), and you could do the same on return by booking the last flight of the night out of NRT (CX501, 9.40pm) and get another 15 hours or so there. At this point you'd probably gladly skip a lounge again at any chance, which is a better way to deal with flying so much.

Then, as I fully agree with the others, you shouldn't try to leave without flying: Nobody at the airport is going to judge you for arriving at opening times (5.30am?), but there are a few things to consider:

• Presuming you continue to fly this much as you have achieved Diamond, rather than another One World Emerald (OWE) status, presumably you will be spending a lot of time in these HKG lounges anyway - so enjoy this special moment, but it will become common
• After the breakfast service is done, there is only so much alcohol and food one can really put into a single body, and you don't get a breakfast,lunch,dinner,evening set-up from CX - just breakfast and "after"
• You really should use your status by getting to enjoy the on-board-perks, the flights can be fun: OWE recognition, worth it?
• Even more so, you'd be better bringing a +1 guest, to the lounges to keep you company - yes you can chat to the Cathay staff, but you can hardly expect an in-depth conversation