What Up That?—Yo.

My brother-in-law said this yesterday at the Circle-Y barbecue. It made me laugh. The phrase has just enough words to convey something, yet not enough of them to resolve the ambiguity. Add the street slang (he’s a law officer) coming from a white boy (though, from New Jersey) and somehow it just stuck with me.
What up that?—Yo.
The punctuation is important and I’m not sure I’ve got it exactly right. The first three words (What up that?) need to convey disbelief, the observational shock of the instigating incident. The italics emphasize the implausibility of the situation — its un-heard-of nature or characteristics. It is posed as a question, indicating an attempt to grasp the surprising or unfamiliar.
But the final word (Yo.) quietly expresses a slow, sad, shake of the head; a smirking “I knew it would turn out this way,” and a superior arch in the eyebrows. This might be followed by a short sigh, with a second shake of the head, perhaps closing the eyes briefly, as if to mourn the dumb-ass under observation. The closing period carries the finality of judgement. Appeals are heard at the discretion of the speaker.
Connecting the two utterances with an em-dash (as above) attempts to unify contradictory – if not schizophrenic – ideas into a single sentence. The two phrases are connected, in that one prevents the other, and vise versa; but also, simultaneously, one requires the other to exist. In this view, the connecting punctuation (which probably has a technical name from grammar class [Notio: “Are there rules for this kind of stuff?” Meg: “No, you had to have been paying attention in elementary school.” {grammar updated based on a comment; which also gave me the chance to use a third level of parentheses}]) – the connecting punctuation carries the flow of the whole expression.
Bringhurst says, “Use spaced en dashes – rather than em dashes or hyphens – to set off phrases.” [Aside: the definition brilliantly models the correct behavior.] “What up that?—Yo.” has two phrases, but that doesn’t quite describe the phenomenology upon hearing them. Bringhurst provides a second clue: “Use the em dash to introduce speakers in narrative dialogue.” Ah, perfect! There are two phrases, so we should separate them with an en dash surrounded by a normal word space. But the phrases are contradictory – as if they were spoken by two different people – therefore we should use a closed em dash—like this.
Ideally we’d follow the em dash with a thin space (M/5) but we’re writing with web fonts; we don’t get M/5 thin spaces this decade.
So that’s one take on it. How else might you punctuate this construction?
Update: The jury of our commenters has specified the correct punctuation as: “What up that? Yo.” It’s settled. If you ever need to write that phrase, that’s how you do it.