Call Amex to confirm, but I would expect since one pays a fee for the Plat Supplementary card, their access would be free too, based on the package of services that have been paid for.

As for the other airline lounges [NW, DL, CO, and the variants based on local country card programs] my understanding is that access is only provided when flying those particular airlines, not unrestricted.

Also note that if you are on one of the EZE flights that has an hour stop at SCL, where you are offloaded for 45-minutes, there is no lounge that can be used by AC Es and SEs or J customers. Even though AC does have a contract lounge for passengers boarding at SCL, it is not open to "transit" passengers. Myself and another passenger checked out every lounge at that airport in search of one that would let us in on this brief stop but had no luck. I could get us into the lounge used as the PRIORITY PASS lounge, but it would have cost U$26 each, and for a cup of coffee, danish and orange juice, neither of us thought it worth the outlay. And by the time we had done our walkabout of the SCL terminal and tour of lounges, it was time to reboard...