Since this is in a restaurant, and a drink taken prior to a meal is usually referred to as an aperitive, why not go with this. Hope it helps.

Anything that is "d'appoint" would be characterized in English as extra, additional, optional, or supplemental. In this context, the waiting-area bar may be an annex to the regular bar (where, presumably, the patrons are not waiting to be seated for dinner). == However, "aperitif bar" (note the preferred spelling of the first word) is a more accurate socio-cultural rendering than "bar annex."

It depends on the quality of the restaurant. If it's a normal restaurant, "bar" is sufficient (e.g. "Would you like to wait at the bar until your table is ready?"). At a better restaurant, the maitre d' would refer to the "lounge", less frequently, "cocktail area", or sometimes though not frequently, "cocktail lounge" (area where music might be played while one sips a cocktail prior to dinner, or simply goes for a cocktail/aperatif).
Bon apetit!

This may be the term you are looking for, again as others have noted, depending upon the quality of the restaurant. The main purpose of a salon in better restaurants is for waiting comfortably to be seated for dinner, but of course libations are available to those waiting there. Hope this helps!

In Vancouver, B.C., this is the term used in the hospitality industry for what you describe. However,restaurant customers don't call it that. They call it the bar or the lounge.