Can I SSH pictures directly to the iOS Camera Roll?
Your going to have to change some sort of index file that the iPad uses to keep track of what photos are in the camera roll.  The file is probably in some sort of proprietary Apple format, the details of which are not available to the public.  You might look in to the format of the file that controls the music library, which is also not public but the details are available around the internet thanks to persistant hackers.  I bet the Photos file format is similar, with Albums swapped out for Playlists.  But I'm mostly guessing.

Good Luck!