Simply edit the OPF file directly. calibre is never going to manage all the insanities of EPUB metadata, it is not an EPUB application, but an ebook application.
And just so you know EPUB metadata is not that simple. opf:role was removed from EPUB 3 (see footnote) in favor of an absurdly complicated scheme that was in turn ditched for EPUB 3.1, where they brought back opf:role and as far as I know it is going to change again for EPUB 4. As such I really have no interest in writing a metadata editor for EPUB.
* Removal of opf:role in EPUB 3
http://www.idpf.org/epub/30/spec/epu...dcmes-optional
* And in EPUB 3.1 they got rid of the refines system entirely,
http://www.idpf.org/epub/31/spec/epu...f-meta-refines