I don't think dashes should be mirrored at all however. (Many of my dashes -- for example these -- are quite symmetrical; but others are not -- I sometimes have a closing dash and sometimes not; but Emily Dickinson is really the expert on the use of the dash: http://mith.umd.edu//WomensStudies/ReadingRoom/Poetry/Dickinson/god-gave-a-loaf. There's no real reason for mirroring these.)
Btw I've /once/ seen directional dashes, in a book of Luciano Canepari. He has a good sense of semantics, in addition to his good phonetic ear :-) , and he does his own things in terms of punctuation anyways, but I very much like it and his ideas. His usage is to have differential spacing -- like this -- or --like this-- (I forgot which). Dunno how common that is in the world's typographic traditions.
About the Emily Dickinson text: parenthetical dashes are in English almost never adjacent to commas (as if there were a constraint against such juxtaposition), though in German one can have a comma after the closing component of a parenthetical dash pair (with no space in between, btw). So those next to commas are dashes of omission or hesitation and not parenthetical ones. Dashes of omission or hesitation aren't paired. (Your quote above implies you're talking about paired dashes, and I'm saying that the poem you linked to has lots of non-parenthetical dashes.)
Stephan
