i have noticed purchases of lounge access at Thai Airways Lounges, by both thai and foreigners, who were on Thai Airways economy tickets.
i would assume they accept purchase at the C/E gate lounges, i would not try this at the business/first lounges at D concourses.
go to the C or E gates Royal Silk Lounges, ask friendly, bring cash.
THB or USD, i think. no credit cards.
sorry I do not remember how much those people paid that i noticed purchasing.