The error you are getting is because the model is unconstrained when you alter the MeasureOrganizer parameter. By default this driver constrains the first and fift ({1,5}) measure and that is the linear y component and rotation around the z axis. When you change it to be the linear x component you need something else to constrain the y direction. Furthermore, there is another driver that already constrains the x component and you will need to change that as well.
Since you want to simulate external accelerations there are other options for you.
You can alter the gravity vector for the collective center of mass as described here Multiple Gravity Vectors - #4 by laozilaozi
Or you can use a moving reference frame to simulate it. It is a bit tricky but I think this will give you the best result. You can practically simulate a roller coaster ride with it. I uploaded our demo model on how to do it and i hope this will help you move on. It might be worthwhile to start with a simpler model than the SeatedHumanFullNeck there are a lot of constraints and parts to keep track on in that. Demo.MovingInertialRef.zip (971 Bytes)
Best regards,
Bjørn
AnyBody Technology