Even when when the HTML files are opened in a browser though, they seem to not have working back-links. Maybe when BookDesigner does the output, back-links aren't saved for HTML files. I really don't know much about the coding side of things, so you'll have to excuse my ignorance on all of this.
This is how a header appears in Sigil in code view, for a book that has chapters simply titled as "Part 1, Part 2, etc.". Hope this is what you meant:
<p class="MsoNormal sgc-4"><a class="sgc-13" id="BdToc_1"><span class="sgc-5" id="title">Part 1</span></a></p>