If you want to include an identifier *for a bibliographic item* (not an author, publisher, etc.)in a <biblStruct>, it's not clear how to do it, and the content models of these elements seem less aligned than you would expect.
Plausible locations for an identifier for a bibliographic item in <biblStruct> are:
a) biblStruct/idno (see the "NISTIR 4681" example at http://www.tei-c.org/release/doc/tei-p5-doc/en/html/CO.html#COBICOI )
b) biblStruct/monogr/idno -- This could contain an identifier relating to the monographic item (such as an ISBN).
c) biblStruct/analytic/idno -- This could contain an identifier relating to the analytic item.
d) biblStruct/series/idno -- This could contain an identifier relating to the series (such as an ISSN).
It would be good to have clarity on choosing between (a) and the others, especially (b) since <monogr> is required in <biblStruct> anyway.
See also http://purl.org/TEI/FR/3555190 for a similar problem with <biblScope>, which can also occur in various places.