Locking Relative Velocity on a Spline

What I'm trying to do is create a sort-of 2D Platformer controller that moves on a 2D plane relative to:
A) where the player is in terms of the spline positioning
B) the angle at which the Player rests relative to the normal on the ground

Given the screenshots, I've managed to isolate the relative X,Y, and Z directions that I want, I just can't seem to find the right method to:

1. Lock the player in place along the X (Red) line
2. allow relative gravity along the normal (Green) and direction (Blue)) (both relative)

I'm thinking that it might have something to do with Vector3.ProjectOnPlane, but I don't know the rest.

Code (CSharp):

1. lineForward = (spline.GetPoint (spline.CPOB (transform.position, 0, 10, 10000) + 0.01f) - newPos).normalized;
2.  
3.         newPos = spline.GetPoint (spline.CPOB (transform.position, 0, 10, 10000));
4.  
5.         Debug.DrawRay (newPos, lineForward.normalized,Color.blue);
6.         Debug.DrawRay (newPos, -Vector3.Cross(Vector3.Cross(currentNormal, lineForward), lineForward).normalized,Color.green);
7.         Debug.DrawRay (newPos, Vector3.Cross(currentNormal, lineForward).normalized,Color.red);