this sounds too easy to be true, but its worth a shot. ive had friends test it in my game and it worked, we opened the chest a few hundred times. from what ive gathered if you join someone's game (cant be host) (game has to be full) and you have a single golden key. you can go to the chest, open it, and once it says you have 0 keys,continue to open it as much as desired.
now i personally have no keys on my online GPD anymore so i cant test it out. I would very much appreciate if someone would try this and tell me what happens. In the event of failure, you will lose your golden key, so be prepared for that. Let me know how it goes.