Mobipocket format defines specific 'guide items' that you put into the .opf file, one for 'toc' and one for 'start'. See:
http://www.mobipocket.com/dev/articl...=opfguides.htm
calibre will generate the 'toc' guide item if you have it generate TOC, I'm not sure what it does for 'start', but you might ask on the calibre forums here. You could also try using kindlegen to convert the epub after adding the guide items - seems like that should work.