Internally Calibre does everything on an epub interpretation of your document. As the tag in question is outside of the spec, it is disregarded.
E-Book and HTML Formatting Rules of Thumb
has some alternative methods. Personally I prefer to just work in Sigil and place each chapter into its own file. It's also an all round easier way of doing things. Since Sigil outputs well formed epubs, Calibre makes it easier to work from, to convert to other formats and such - an all round easier format.