If iBooks Author allowed you to embed the HTML code (available below the photosynth or panorama which you're interested in) then the pano or synth could load as long as the book owner had internet access.
This is not an official answer, though.
If it would require special co-operation between Photosynth and Apple to get this to happen and download all the necessary viewer files to insert inside the book file, then this is less likely to happen, just because it seems as though Photosynth almost never gets developed on anymore, but perhaps that's a poor assumption on my part.
The other downside to downloading all the viewer files to insert inside the book file is that you're going to increase the size of the book by hundreds of megabytes for a high resolution panorama or several gigabytes in the case of some of the largest photosynths. That kind of file size increase just so that the pano or synth works whill offline is too much for most, not to mention Apple.