IIRC While it is typically reserved for the imperitive “All y’all” is not used in a subordinate clause.
One would not say, for example, “Abandon hope, all y’all as comin’ in.” but “All y’all best not be goin’ in there.” would work. Still, strays a bit from the raw Dante.
I freely admit to being a semiliterate thug, but doesn’t the formality of writing increase as one reads from Inferno to Paradisio? If that’s the case, it would make sense that the sign above the gates of Hell would be scrawled in some imprecise vernacular drivel.
I’m not familiar with that play (I’ll bet Shaw wasn’t either, but the man was phenomenally well-read). I’ve always thought that his inspiration was chiefly from Mozart’s opera. Shaw liked nothing better than taking our usual expectations and turning them on their heads. Look at his Caesar and Cleopatra, which totally manages to avoid any romantic entanglements.