I don't really know enough about Latin morphology or grammar to get a clear cut answer (already knowing that Latin is a very free-form language), so how does one say it? I was thinking something like trans aulaeum or maybe ultra aulaeum but I don't know if Latin allows for stand-alone prepositional phrases like English can.
Latin certainly allows “stand-alone prepositional phrases,” and I'm sure you already know a few of them, for example:
- ante meridiem, post meridiem (a.m. / p.m.)
- ad hoc, ad libitum, ad inifinitum, ad nauseam
- in absentia, in medias res, in memoriam
- de iure, de facto
- ex cathedra, ex post facto
So I see no problem with ultra aulaeum, for example.
poscaenium ~ poscēnium is the name for the place behind the wall of the stage, and is probably what you're looking for, e.g. in poscēniō. It's derived from the phrase post scaenam ~ scēnam "behind the stage". The Roman theatre only had curtains in front of the stage that were lowered to reveal it, so the service space that for us is behind the curtains was for the Romans behind the stage decorations.
ultrā "further beyond" and aula "the (theatre) hall" don't work to express this; but the curtain revealing the stage was called aulaeum, so post aulaeum would express the opposite place from what is intended, i.e. the stage itself with decorations.