[KevinH]

I really have no plans to support epub3 prefix and new style meta tags in an epub2 opf as Sigil itself does not support them (and the opf must be machine readable and grok-able at all times inside Sigil to allow things like automatic splitting, file renaming, etc).

The simplest solution is to open the epub3 after conversion in Sigil and manually add them using a clip or search and replace, or whatever is easier.

Or ask someone to make an epub3 compatible iBooks plugin adapter and run it in Sigil after conversion.

Or hard code your own copy of the epub3itizer to handle your special case for ibooks.