If I understand correctly that the restaurant is paid a flat fee per person, the same as a lounge, the transaction amount wouldn't need to be passed to Priority Pass. I hope that the Priority Pass system would reject the card in real time.
My sense as well. So if my guess is right, an Amex-issued PP card won’t work at all at these restaurants. The cardholder might have a restaurant bill of $5 or $500, but there’s no way for the restaurant to send that information through the system.