AA starts DFW-HKG on June 11. You'd need to backtrack to DFW to upgrade on AA metal as they do not offer service from LAX. You can't use AA miles to upgrade the nonstop LAX-HKG on Cathay.

Two issues:

1. AA caps the amount of miles you can buy in a year at 60,000 per member. This should only impact you if you've already bought miles.

2. AA controls the inventory and the chances of finding 2 seats on the same aircraft, particularly on a new route that might be in demand, may be slim. You could end up buying all these miles and not have any seats to use them on. I'd check out availability before making major purchases of miles that you might be stuck with.
AADVANTAGE PARTICIPATING AIRLINES UPGRADE AWARDS
AA upgrades