a "sas" would be a small area like a passageway between the room and the garden I think. Maybe doorway would fit here? On another topic, the word "habit" is generally used when referring to the attire of monks or nuns (monk's grey habit)
HTH

the sas, as a passageway or entryway,could refer to that part of stage left (côté jardin) that is the transition from off-stage to on : the wings


Grey monk's habit - wing stage left

I think Matthew has probably got it right with 'wings' (NB: plural, even just for one side of the stage!), but according to this little doc I discovered on the 'Net, it just might be 'scene dock' — it's not unusual for this space to be used for quick changes, rather than going back to a dressing room.

Activité

Avoir un espace de dégagement (sas) pour les décors et les accessoires. ... Disposer d'une scène (dont coulisses) plus large que profonde, ...

concertationcroissy.blogs.com/concertation_chateau_croi/files/crgtcroissy_20407.doc

But I'm still concerend that the caps might indicate an acronym and I was trying to think of one, like for example 'Salle : Avant Scène' — the costume to be kept ready on the forestage FOH.