Originally Posted by bfollowell
The only way I can see to make it work is to rename the epub as a zip, pull out the opf and add the line manually to the guide section, then add the modified opf back into the epub and convert. That does work perfectly mind you, it just seems a little more cumbersome than it should be.
I know very little about mobi, but if you're a Calibre user you can make the above slightly less cumbersome by using the Tweak-epub option. It will explode the epub for you. You still need to manually edit the .opf file. Then Press the Rebuild button to rezip it.