I can’t guarantee that this will work always, but I did just experiment today with adjusting the start position of a kindlegen-converted EPUB3→MOBI file. In the nav file, landmarks section, I put
Code:
<li><a epub:type="bodymatter" href="../Text/Preface.xhtml">Start</a></li>
—and it seems both the “bodymatter” and “Start” were needed (this file originally had its type set to “prefix”)—and in content.opf’s guide section,
Code:
<reference href="Text/Preface.xhtml" title="Text" type="text"/>
—again, type="text" seemed to be required. (I’m not sure about title="Text"; I’d previously tried writing title="Start", but Sigil kept rewriting that to title="Text" so I gave up. This works, and I leave to those more adventurous to test the limits of what else might work.)
The kindlegen’d file opened correctly, both in the Kindle Previewer and in my Kindle Fire HD, at the correct page.
ETA: Of course, what Amazon does to the file afterward is beyond anyone’s control.