Comply with local regulations, which will likely mean opening at greatly reduced capacity, with no self-service food or drink.. ... I would expect temporary elimination of guests, shortening of the three hour before departure window, and perhaps a ban on entry too close to departure.
Buffet-style food is going to be an issue. As to doing the capacity-reducing things above, I don't think they'll be necessary. Airlines are talking about operating at 30% of their normal schedule for the rest of the year. That alone is more than enough to solve the overcrowding issue. And, if the buffet is gone, that's going to reduce demand from those that are left. Of all the travel and Amex problems we have to look forward to in the next year, overcrowding at the Centurion lounges isn't one of them. It's right up there with worrying about how the continued grounding of the 737-MAX and the pilot shortage is affecting airlines' ability to operate their schedules.