With some more research I found a library Canon released for developing USB camera control software. - Wow, more feature creep already. That lead me to a linux application called gphoto2, available as an apt-get download. The project will stay with the 2.5mm jack control for now, but eventually I'm going to add in access through USB. Especially since USB access has the promise of putting a live image on the pi's monitor. Now, at the moment the project is headless, but a nice dim monitor with a live image from the camera is very useful for aiming , focusing and composing shots.
The screen on the back of a camera is useful, but, the menus are horribly bright for night time work, and the camera can get into some unusual positions. It's pointing up at the sky, to check the monitor you need to get under that screen, and not bash into the tripod. With a darker desktop, a small computer screen would be great for monitoring the camera, doing in field code mods, and using other Linux astronomy apps. I've added photos to the project that show how well the desktop of the Pi can be darkened. I think I'll have to get into some text files to go further, and edit the icons.
These mods are just some low hanging fruit, but they will come in very handy after the main program and board are debugged. But seeing ready made USB camera software opens up a lot of possibilities