IIRC Steve2 asked a Rav who said that if you show them your wallet with cards in it and ask if any of these work to get into the lounge and if they point out that the Plat card gets you in then it's OK.
I may be misremembering the exact details though.
Either way, congrats on the free miles! Please post a comment on the main site about it on the AMEX Plat post.
You must have some details messed up because there's no way that would work. Everyone would agree that to use counterfeit money is asur, but if I walk into a store and show them my wallet and ask if any of these bills work and they say that 100$ counterfeit bill would work, that would be ok?
I asked this more than once, more than twice, to try and find an opinion that would permit this (not for myself as I had already gotten my psak of asur, but for a limud zechus on everyone else), and was unable to find any who agreed once they heard all the details clearly. (some even suggested it's has more than one issur deoraisa involved) Either way, even if I would find that one Rav who would permit it, can anyone really feel secure being someich on a random rav who is not their own, going against the majority of the rest of the rabbanim? Either way, I am still going to search for the limud zechus, and I'll PM IIRC Steve2 to see who he asked and exactly what they said. I guess I'll keep everyone posted.