The Edit in Sigil would probably be useless, but it could be use changed to more easily send to third party editor. But the whole program will still have a use, the Table of Content editor for example.
Why don't you have a try at it with Mono? the split chapters will probably not work because the dll is native for windows (you could just replace it with the MAC version of Tidy). I think the preview might not work on MAC. BUt maybe someone could test it, I would love to have some feedback about OS X / Linux compatibility.
ePubFixer - http://code.google.com/p/epubfixer/
Edit your TOC, by Adding, Renaming, Shifting Chapter, Change chapters order or nest chapter into Part or Book easily by Drag and Drop. Split files by Chapters & Auto Detect phrase for your chapter heading. Also edit the reading order, fix margins & Edit your file with Sigil and restoring your previous TOC that was lost. Edit your cover without reconverting the file, Create an Inline TOC