Well, if it is so easy to implement, why don't you do it?
Sigil is intended to become EPUB3-aware at some unspecified future time. The appropriate time for EPUB3-specific features to be implemented is as part of that release.
It is silly to use EPUB3-specific features in an EPUB2, and since Sigil officially deals with EPUB2 alone, it would be doing a disservice to Sigil users to make them think EPUB3-specific features are part of the standard Sigil featureset -- they might accidentally use them.