Sigil is still being developed, but it should be noted that contributions are on an as-offered basis.
Currently @KevinH has been spending quite some time doing a considerable amount of work on Sigil. If and when he is satisfied, there is no guarantee he will stick around.
The only person guaranteed to continue focusing on Sigil is @user_none, who operates in management-only position -- he hosts Sigil, controls what features get added, and accepts other peoples' contributions, but has no time to focus on adding anything himself.
I can see this kind of capability in Sigil, it is within the purview of a WYSIWYG editor.
calibre expects users to work strictly in the code, and automated munging of the XHTML in order to split at arbitrary locations will most likely not happen.
Although... calibre's editor supports plugins, which could perform all sorts of automated hackery that won't be supported in calibre itself.