If I understand you correctly, you want to have a toc that that lists the chapters (e.g. Chapter 1, Chapter 2, etc), but you don't want to have this shown on the actual page, just have a * as the displayed chapter name.
What you could do is add a title to each of the * headings like the following example:
<h2 title="Chapter 1">*</h2>
This method will tell Sigil to use the text in the title tag for the toc and not the actual displayed text.