Granted, it has little to do with the epub spec per se
. But boy-o-boy would it be useful. If I could get my calcified ancient brain to wrap around xml and ONIX specs at the same time, I am sure I could write a PERL script to convert the <metadata> section of content.opf to an ONIX file. (This would assume, of course that all the pertinent data were contained in the <metadata> section.) I've thought about it many a time, but my afore-mentioned ancient brain turns to peanut butter whenever I try.
Edited to add: Here's why it would be useful, for those who are not submitting books to various distributors such as Sony, B&N, etc. just so you know: One doesn't just upload the epub file; one also has to provide all the metadata for the book in a separate file which, typically, is in a proprietary excel spreadsheet format, OR as an ONIX file. So if one could create an ONIX file from the epub's metadata, one would save a lot of double-keyboarding. Or multi-keyboarding, given that the proprietary excel spreadsheets, of course, differ from each other though containing the same information.
FWIW, Sony reportedly has a MS-Office excel spreadsheet macro to convert their spreadsheet format ( as used to submit books to Sony, and provides the same information as an ONIX file would do) to an ONIX file. We haven't fully tested it yet, but it looks promising. If it works reliably, we will be shifting towards storing all our metadata in sony .xls format for later conversion to ONIX, for submission to other distributors that accept ONIX data in lieu of proprietary spreadsheet formats. But wouldn't it be great if one could extract the data from the epub itself! Calibre can already extract it to text. Conversion to ONIX should be do-able, yes?
So if any of you young'uns feel the urge to write and share such a script, i.e. <metadata> --> ONIX, (PERL is fine by me, but python might be a more widely acceptable choice) please do so. Hopefully in the (not too distant) future, Sigil will have plugin capability. That would make it easy to add such a capability, for those who want it. (hint, hint)