// Generate planar uv coordinates for the secondary uv set

function Start () {
    var mesh : Mesh = GetComponent(MeshFilter).mesh;
    var vertices : Vector3[]  = mesh.vertices;
    var uvs : Vector2[] = new Vector2[vertices.Length];

    for (var i = 0 ; i < uvs.Length; i++)
        uvs[i] = Vector2 (vertices[i].x, vertices[i].z);

    mesh.uv2 = uvs;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        Mesh mesh = GetComponent<MeshFilter>().mesh;
        Vector3[] vertices = mesh.vertices;
        Vector2[] uvs = new Vector2[vertices.Length];
        int i = 0;
        while (i < uvs.Length) {
            uvs[i] = new Vector2(vertices[i].x, vertices[i].z);
            i++;
        }
        mesh.uv2 = uvs;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        mesh as Mesh = GetComponent[of MeshFilter]().mesh
        vertices as (Vector3) = mesh.vertices
        uvs as (Vector2) = array[of Vector2](vertices.Length)
        i as int = 0
        while i < uvs.Length:
            uvs[i] = Vector2(vertices[i].x, vertices[i].z)
            i++
        mesh.uv2 = uvs