Yes, I believe that we can add this behavior. The best thing to do is honor the toclevels (i.e., toc depth) attribute. I just need to see how Kindle deals with the extra levels (since it has a unique way of displaying the TOC).
Before we continue, I want to correct the terminology being used here. The entries currently shown in the TOC are the chapter titles. Each chapter is packaged as a separate HTML file in the archive. The TOC is essentially giving a list of these chapter files. What you're asking to be added to the TOC are the section titles (or headings).
We'll call what is shown right now as toclevels=1. If you want the top-level sections in a chapter to show, you'll set toclevels=2, and so forth.