Our experience is precisely as Hitch described.
AFAIK, no aggregator or distributor digs into the epub metadata for anything.
That said, if I really really wanted to include BISAC codes, or their human-readable equivalents, in the metadata, I would simply put it within the <dc:subject> tags like any other keywords.
We do use the <dc:subject> tags, although I am unaware of any software that makes use of them at the moment. Maybe Calibre? Kind of like the Voyager project; we send it out there, not knowing who is listening....
Albert
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