[kovidgoyal]

The hardest part of writing a sigil replacement inside calibre would be the editor component, especially if you want both a code editor and a WYSIWYG editor.

The rest: editing a list of files, editing metadata, editing ToC, renaming/moving around files would be trivial.

So to move this conversation along, here's a concrete offer. I offer to implement the *rest* part of it -- list of files, edit metadata, edit tocs, renaming/moving files, editing the spine, various automated actions from the Polish Books component -- if someone will volunteer to tackle the editor component.

I can set up a basic GUI shell that does most of that in a few weeks, and then let whoever volunteers work on the editor component. But, I will only spend the time to do that if I get a firm commitment from someone to handle the editor part. That committment will need to include ongoing support for fixing bugs in the editor component.