I have 2 First Classs award tickets HKG to JFK on CX 840 which leaves at 7pm. The flight gets in to JFK so late that I will have to overnight in NYC to catch a connector. Considering these are award tiks and there are no seats available on the earlier flight available, would I be able to standby for an earlier flight at the airport?

Don't see why not. I did this on an award seat out of Hong Kong just last week (although it was on Dragonair rather than Cathay) and have also done it ex-Bangkok. Not sure if you can do it at the intown check in. If I was you, I would head directly for the airport. I timed it so I got there shortly before check in was about to close, by which time they had a pretty good idea there were a couple of spare seats and (after a brief phone call to a supervisor) the check in clerk released a seat to me.

This is one thing I really like about CX... it costs them nothing to allow you to fly earlier, and generates a lot of goodwill.

I've done this a couple of times on both award and paid (with and without charge). It's usually easily done, but have heard both views on the reissue charge. I was twice in London trying for a Y standby and both times they tried to charge me the reissue charge. They just try to put that on me for whatever reason.
However, also a good one also was when i was in London with my parents last week- had to leave 1 flight earlier for family stuff but Y was overbooked and J was soldout. They managed to get us 2 J seats and a points upgrade to F. So happy family for that.
HKG-TPE-HKG are quite easy to be done... I would imagine that to be the same for other short-hauls esp with status.

I did it once. I was confirmed on 881, but went to the airport a little early, in hope of catching 883 (2 hours early). It was done with any problem/surcharge.

Do you mean "It was done without any problem/surcharge"?

Depends on the type fo ticket you have, if it is a highly restricted Y Class tix then of course they can surcharge you or refuse to standby. If it is flexible ticket they you can Standby all you want without penalty.

Great, I think I will do that, hopefully they will have the 777 on the other JFK direct that day, heard they have it on the earlier direct 3 times a week.

If you are trying to standby for 830, be aware that 830 is very popular with premium flyers. It is rare that 830 will have two empty F seats.

You are very lucky. It is very difficult to grab First Class reward on JFK flights.

I think you'll be fine ^

Another question about standby on "CX" flight
I have a ticket from ORD-LAX-HKG-TPE.
ORD-LAX segment is on CX flight number operated by AA.
Can I do a standby on same day earlier flight?

Reward tickets are easily rebookable without a penalty or fine, and you can use these to stand by. I've never been on standby on CX, but I have done so once on Aer Lingus (when it was OW), and twice on BA, and all 3 times I got my seat.

On Aer Lingus I stood by for an earlier flight, and it was no problem.

On BA I stood by once for the next flight as my incoming flight was delayed due to weather and I missed my connection (therefore I no showed for my actual flight). They put me on standby and gave me the last seat in J.

The second time in BA, I issued a ticket on a different date in J, and then rebooked my date & downgraded myself to WT+ class on the day I wanted to travel (the clasee between economy and business), I did this as J wasn't available. Then on the day itself at the airport, I asked them to get me a seat in business class as my ticket was issued as a J ticket, they put me on standby and got me my seat. It was a gamble, but it paid off.

My advice to you is to study the loading before you go to judge if its worth it to try.

I'm ticketed on CX872 for 2 pax in J 5/27 on an American Award ticket and wanted to get on the earlier CX870 as it is planned to be operated with the new seats. Is it any more difficult to standby with an AA award ticket? I'd really like to try the new seat.

There is no guarantee that one is a 74A and the other not. The two flights are only two hours apart. You can go to the airport early and check on the equipment (which has the 74As) and whether you can change to 870. In the worst scenario, you will just end up spending another 2-3 hours in the CX lounge(s).