You just can’t make nospace-emdash-nospace work in the long run. There are too many edge cases that become incomprehensible, as in this Report on Business cover (April 2007):
Rewritten:
Issy Sharp’s five-star,
platinum-plated,
exquisitely tasteful—
and Saudi prince–
endorsed—quest for world domination*
Note that “Saudi prince–endorsed” must use an en dash and doesn’t. They hang the apostrophe, but do nothing about the fact that five of the six lines have some kind of hyphen or dash. Why they didn’t break up the appositive phrase “and Saudi prince–endorsed” onto its own lines is beyond me.
Sorry, kids. You have to use space-endash-space. You can reserve em dash for introductions of dialogue, cut-short interruptions, and the like.
(There’s a similar example in Spy: The Funny Years, actually.)