Use any camera app with the lock screen camera button
With AnyLockApp you can easily open any app from the camera button on the lock screen. Exchange default iPhone camera with Camera+. No problem. The app works as advertised and is free.
Not available for regular iOS users.
(There is currently a bug preventing the app from working when a passcode needs to entered. I’ve contacted the developer for a fix. For the moment, if you’re stuck at the lock screen. Hold the power button for a short moment and then tap cancel. This will take you to the screen where you can enter your passcode.)