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.