Hmm I checked Doitsu's Epub2LegacyMetadata plugin and in that he uses this style:
Code:
<dc:identifier opf:scheme="MOBI-ASIN">B005IEGK5C</dc:identifier>
So it would appear that some e-readers or calibre can grok that format as well in epub2.
The spec's flexibility here (where none is truly needed) makes this a bit of a muddle.