From version 5.3 (beta available here), you'll be able to modify a .sh3d file more easily because this zip file contains a new entry Home.xml entry that can be edited with a simple text editor.
If you look into this Home.xml file, you'll see that that camera positions of the video are stored in a list of <observerCamera> tags (or <camera> for aerial view positions) with an attribute equal to cameraPath, as in the following example: Feel free to edit these lines or even add some new ones. x, y and z attributes are in centimeters, yaw, pitch and fieldOfView attributes are in radians and time in milliseconds since the Epoch (1st of January 1970 UTC). See SweetHome3D.dtd for more information. Of course, be sure to backup your .sh3d file before trying this kind of changes.