If you have the width, can you not just use that to create a temporary object at that distance either side of the centre, fire the ray from that and then delete it? Or alternatively keep the objects there to save re-creation, depending on how often it needs calculating.

I haven't tested this code but I think this method should work,

var characterController : CharacterController;
var characterController = gameObject.GetComponent(CharacterController);
var characterRadius : float = characterController.radius;

var rayOrigin : Vector3;
var hit : RaycastHit;

// Right edge forward facing raycast
rayOrigin = transform.position + transform.right * characterRadius;
if (Physics.Raycast(rayOrigin, transform.forward, hit, 10)) {
    // Executed on right side hit
}

// Left edge forward facing raycast
rayOrigin = transform.position - transform.right * characterRadius;
if (Physics.Raycast(rayOrigin, transform.forward, hit, 10)) {
    // Executed on left side hit
}