In response to the question "Where is she?", I've heard someone say, "She's back the hall." (Cf. "She's back there.") I understand the meaning to be something like "She's down the hall," "She's in the hall," or "She's in some room connected to the hall."
It seems that back is functioning as a preposition here, as it doesn't quite fit the adverb usage in its entry at M-W.
Is this a common usage in the sense of being documented in a reliable reference work or documented as an idiom or variant outside normal usage? If so, what part of speech is back?