I like the idea of adding additional sensors, but how would you mount the tilt sensor to detect movement? Maybe you could just activate the accelerometer function of the LSM303 in the code?

It doesn't matter how slow they go, if the backpack is rotated, the alarm will sound. A smart thief who knows how the alarm works and tries not to rotate the backpack will bypass the security. You could adjust the sensitivity to make sure that the security is much more difficult to bypass. However, most thieves probably won't expect an alarm in a backpack, so they won't be as careful.