It seems basic, but I am pretty sure that is the expression I have heard often in such circumstances.

D'appoint literally means "support or supplementary", so what we have here is a secondary bar. In the UK the equivalent would be known as the "lounge bar, although in practice it is often just called the bar.

Hope this helps.

I've been asked repeatedly to wait in the lounge, or to wait at the bar. I've never specifically heard waiting bar and waiting lounge only at an airport. But depending on context, it could be helpful to include the word "waiting", e.g. for distinction from another lounge or bar.