When airline lounges first started in the USA, they were private clubs and as such the traditional was not to tip in such places in the USA, although there may have been service charges added to bills or special gratuities for the holiday season, organized by club management.

As airline lounges have evolved, IMO it's an ambiguous situation. I wouldn't tip the reception and customer service lounge staff, who are almost always airline employees (just like one doesn't tip a FA or GA). Food service workers are likely to be contract employees (with few employee benefits) who are paid close to minimum wage, perhaps the minimum wage for tipped employees. The same may be true for those who staff the shower areas. Still, in most lounges, you don't pay for what you consume, nor do you ever see a price list (buffets and menus don't list prices for what you order), so you can't start from the 15% benchmark and adjust accordingly. [An exception might be if you choose to pay for "luxury" alcohol that's offered in some lounges.]

For most alcoholic drinks, I would tip $1 per drink to the bartender or server, but nothing if it's self service. For a meal that's served in a dining room area, I'd probably tip a couple dollars (like I would in a buffet) for pretty good service, maybe about $5 per person for excellent service if you only ordered one or two items (more for an extensive multi-course meal with excellent service), and nothing for bad service, but there's no real guideline and of course in an international airport, there's the added complication that some guests might not have any local currency. If your goal is to tip as if you were in a real restaurant, you could estimate the cost (not including tax) of your meal and tip about 20% of that for very good professional service in the NYC area.

This is my interpretation of American rules. I would not tip in Clubhouses outside of the USA.