Hi and thanks for noticing.
Making use of the brake pedal is not an option since the device should be as unobtrusive as possible. Yet, it must be very easy to install. I am planning on building a small box that
a) holds the Arduino board and the accelerometer and
b) connects to a 12 Volts power outlet to provide power for the board and allows for charging an Android handset.
There will probably be a simple push button glued to the dashboard (i.e. being reachable for the driver) to trigger a manual action.
Since we do not have many steep mountains in our tiny country we usually don't drive uphill with 1g 
But you might have a point here. That's why I am asking you guys. I don't have any experience in making sense of accelerometer data.