This video shows how I used a magnetic switch to prevent my 2008 TL with a manual transmission from starting (and remote starting) while in gear. Putting in a remote start the proper way is usually the way to go since its better safe than sorry but I'd prefer not to go through all the steps before exiting my vehicle so I came up with another way. Please do this at your own risk if you decide to do so as I am not responsible for any damage to your car.
The switch I used is a normally closed (N.C.) magnetic (reed) switch. A normally closed switch will have the circuit closed without a magnet. Think of this as a light switch in the ON position. When a magnet is in range of the magnetic switch, it will open the circuit such as a light switch in the OFF position. My original setup was installing the remote start as if my car was an automatic with the neutral safety switch permanently jumped. This was bad because it will start in any situation whether your car is in gear or not. When remote starting a car in gear, it will move forward even with your hand brake engaged. With this magnetic switch, the circuit will be closed in neutral and when its in gear, the circuit will open. The two leads from the magnetic switch goes directly into the neutral safety switch, opening and closing the circuit as if you were stepping on it with your foot. To mount the switch, I 3D created and 3D printed a mounting bracket as well as some for the magnets.
There are many other ways of doing this but I wanted to show you that it is possible. Originally I wanted to use a N.O. switch but ended up getting a N.C switch from incorrect packaging. The concept is the same for many other cars, the main difference is where you would mount it.